Iron Supplements: Evidence Guide to Benefits, Forms, Dosing, and Safety

Iron is an essential mineral required in small amounts for a wide range of critical physiological functions. It is a core component of hemoglobin, the erythrocyte (red blood cell) protein responsible for transporting oxygen from the lungs to tissues throughout the body [1][2]. Iron also forms part of myoglobin, a protein that provides oxygen to muscle tissue, supporting muscle metabolism and healthy connective tissue [2]. Beyond oxygen transport, iron is necessary for physical growth, neurological development, cellular functioning, the synthesis of certain hormones, and the conversion of thyroid hormones — specifically, the conversion of thyroxine (T4) into the biologically active triiodothyronine (T3) [2][3][4].

The human body contains approximately 3 to 4 grams of elemental iron, most of which resides in hemoglobin [2]. The remainder is stored as ferritin or hemosiderin in the liver, spleen, and bone marrow, or is present in the myoglobin of muscle tissue [1][2]. Transferrin is the primary transport protein that binds iron and distributes it throughout the body. Iron losses are normally small — through urine, feces, sweat, and shed skin cells — but losses increase substantially in menstruating women due to blood loss [2]. Hepcidin, a circulating peptide hormone produced in the liver, is the key regulator of both iron absorption and distribution throughout the body [2][5].

Iron deficiency is the most common nutritional deficiency worldwide. The World Health Organization (WHO) estimates that approximately half of the 1.62 billion cases of anemia worldwide are due to iron deficiency [7]. Among females aged 12–21 in the United States, an estimated 39% are iron deficient and 6% have iron deficiency anemia [8]. Even mild deficiency — too mild to cause anemia — can cause fatigue, impair cognitive function, and reduce exercise performance [8][9].

Table of Contents

• Overview
• Forms and Bioavailability
• Evidence for Benefits
• Recommended Dosing
• Safety and Side Effects
• Drug Interactions
• Dietary Sources
• References

Overview

Dietary Iron: Heme vs. Nonheme

Dietary iron exists in two main forms [1][2]:

• Heme iron: Found in meat, seafood, and poultry. Forms when iron combines with protoporphyrin IX. Contributes approximately 10–15% of total iron intake in Western diets but has substantially higher bioavailability (approximately 15–35% absorption) [2][6].
• Nonheme iron: Found in plants, iron-fortified foods, and also in animal products alongside heme iron. Absorption rates are lower (approximately 2–20%) and are more heavily influenced by dietary enhancers and inhibitors [2][6].

The bioavailability of iron from mixed diets containing substantial amounts of meat, seafood, and vitamin C is approximately 14–18%, compared with 5–12% from vegetarian diets [2][6].

Assessing Iron Status

Iron status is assessed using several laboratory markers [2][9]:

• Serum ferritin: The most efficient and cost-effective test for diagnosing iron deficiency. A value below 30 mcg/L suggests iron deficiency; below 10 mcg/L suggests iron deficiency anemia [9][10]. However, ferritin is an acute-phase reactant and can be elevated by inflammation, which complicates interpretation in patients with infections or chronic diseases [9].
• Hemoglobin and hematocrit: The most commonly used screening measures, though they are neither sensitive nor specific. Hemoglobin below 11 g/dL in children under 10, or below 12 g/dL in individuals aged 10 and older, suggests iron deficiency anemia [9]. Normal hematocrit values are approximately 41–50% in males and 36–44% in females [2].
• Transferrin saturation: Calculated in part from serum iron levels. Declines during iron-deficient erythropoiesis [9].

Iron deficiency progresses through three stages [2][9]:

• Storage iron depletion (mild deficiency): Serum ferritin concentrations and bone marrow iron decrease, but hemoglobin remains normal.
• Iron-deficient erythropoiesis (marginal deficiency): Iron stores are depleted and transferrin saturation declines, but hemoglobin levels remain within the normal range.
• Iron deficiency anemia (IDA): Iron stores are exhausted, and hemoglobin and hematocrit decline. Red blood cells become small (microcytic) and pale (hypochromic).

Symptoms of Iron Deficiency

Symptoms range from mild to severe and include [8][9]:

• Easy fatigability, weakness, lethargy
• Shortness of breath, dizziness, headache
• Coldness of hands and feet, pale skin
• Chest pain, irregular heartbeats (arrhythmias), heart murmur, enlarged heart
• Brittle nails, hair loss
• Swelling or soreness of the tongue, cracks at the sides of the mouth
• Loss of taste, enlarged spleen, frequent infections
• Restless legs syndrome — strongly associated with low iron/ferritin levels [8]
• Pica — compulsive eating of non-nutritive substances such as ice, dirt, chalk, or clay (estimated in 11–55% of iron-deficient individuals) [8]
• Recurrent canker sores (aphthous stomatitis) [8]
• Difficulty swallowing — particularly in Plummer-Vinson syndrome [8]

Iron deficiency anemia has also been associated with tinnitus. A population-based study of over 100,000 women in Taiwan found that women with iron deficiency anemia had a 278% increased risk of new-onset tinnitus at 1-year follow-up. Older women (>45 years) were more likely to have common subjective tinnitus, while younger women (18–44 years) had increased risk of both subjective and pulsatile tinnitus (Hung, Front Nutr 2025) [8]. Pulsatile tinnitus may resolve when deficiency is corrected with iron supplementation [8].

Prevalence

• 39% of U.S. females aged 12–21 are iron deficient (Weyland, JAMA 2023) [8]
• 7.43% of U.S. women aged 25+ are iron deficient (Barton, JAMA Netw Open 2024) [8]
• 6.7% of U.S. children aged 1–5 are iron deficient; 11% among Mexican-American children [8]
• Approximately 5% of women and 2% of men have iron deficiency anemia [8]
• 29% of men are at risk for iron excess [8]

Forms and Bioavailability

Not all iron supplements are equivalent. The form determines elemental iron content (how much actual iron per milligram of compound), bioavailability (how well it is absorbed), and gastrointestinal tolerance (likelihood of side effects like constipation, nausea, and stomach upset).

Comparison Table

Form	Elemental Iron (%)	Relative Absorption	GI Side Effects	Notes
Ferrous Sulfate	20%	High	Moderate-High	Most studied form. Inexpensive. Standard comparator in clinical trials [2][11].
Ferrous Fumarate	33%	High	Moderate-High	Highest elemental iron per gram of compound. Commonly used in prenatal supplements [2][11].
Ferrous Gluconate	12%	Moderate-High	Moderate	Lower elemental iron content but generally well tolerated [2][11].
Ferrous Bisglycinate (Iron Glycinate)	~20%	High	Low	Chelated to glycine. Better tolerated than ferrous sulfate with comparable absorption. Less affected by food inhibitors [11][12].
Ferric Citrate	Variable	Moderate	Low-Moderate	Ferric (Fe3+) forms are generally less bioavailable than ferrous (Fe2+) forms due to lower solubility [2][11].
Ferric Maltol (Accrufer/Feraccru)	Variable	Moderate	Low	Prescription-only. Specifically studied in inflammatory bowel disease patients intolerant of other forms [8][13].
Heme Iron Polypeptide	Variable	High	Low	Derived from animal hemoglobin. Absorption less affected by food inhibitors than nonheme forms [11].
Carbonyl Iron	~98%	Gradual	Low-Moderate	Highly concentrated elemental iron but absorbed slowly via dissolution in gastric acid. Lower acute toxicity risk [11].
Polysaccharide-Iron Complex	Variable	Moderate	Low	Iron bound to polysaccharide carrier. May have fewer GI side effects than ferrous salts [11].

Key Principles for Form Selection

Ferrous vs. Ferric: Ferrous iron (Fe2+) has higher bioavailability than ferric iron (Fe3+) because of its greater solubility in the intestinal lumen [2][11]. Most dietary supplements use ferrous forms for this reason.

Chelated forms (bisglycinate): Iron bisglycinate chelate is bound to two molecules of glycine, which protects the iron from interaction with dietary inhibitors (phytates, polyphenols, calcium) and may reduce gastrointestinal side effects. Studies suggest comparable or superior absorption to ferrous sulfate with better tolerance [11][12].

Heme iron polypeptides: Derived from animal hemoglobin, absorbed through a different pathway (heme carrier protein 1) than nonheme iron. Absorption is less affected by dietary inhibitors. May cause fewer GI side effects but tend to be more expensive [11].

Ferric maltol: A prescription form (Accrufer/Feraccru) shown to be tolerable and effective in treating iron deficiency anemia in people with Crohn's disease or other inflammatory bowel disease who were unable to tolerate other forms such as ferrous gluconate, ferrous succinate, and ferrous sulfate (Weisshof, Curr Opin Clin Nutr Metab Care 2015) [8][13].

Carbonyl iron: Nearly pure elemental iron (98%) that is absorbed gradually via dissolution in gastric acid. The slow absorption provides a safety advantage — lower risk of acute iron poisoning — making it the only form exempted from child-resistant packaging requirements above 30 mg [8][11].

Factors Affecting Absorption

Enhancers of iron absorption:

• Vitamin C (ascorbic acid): Converts ferric iron to the more absorbable ferrous form and helps overcome dietary inhibitors. Taking iron with orange juice improved absorption in young women given 100 mg ferrous fumarate (von Siebenthal, Am J Hematol 2023) [8]. However, a study of 440 people in China found that taking each 100 mg dose of iron (ferrous succinate) with 200 mg vitamin C did not produce a significantly greater increase in hemoglobin or ferritin at 2 and 8 weeks compared to iron alone (Li, JAMA Netw Open 2020) [8]. Based on this evidence, the British Society of Gastroenterology does not recommend routinely taking vitamin C with iron (Snook, Gut 2021) [8].
• Meat, poultry, and seafood (the "meat factor") enhance nonheme iron absorption [2][6].
• Gastric acid is essential for nonheme iron absorption — converting ferric to ferrous iron [2].

Inhibitors of iron absorption:

• Phytates in grains, beans, and legumes [2][6]
• Polyphenols in tea, coffee, cocoa, and some vegetables. Green tea and coffee may reduce absorption by up to 50%, though drinking coffee at least one hour before iron supplementation does not appear to affect absorption [6][8].
• Calcium may reduce absorption of both heme and nonheme iron, though this effect is attenuated in a typical Western diet [2][6].
• Proton pump inhibitors (PPIs) such as omeprazole and lansoprazole reduce gastric acid, impairing nonheme iron absorption [2][14].
• Grape seed extract polyphenols can bind iron and inhibit absorption (Ma, J Food Sci 2011; Mottaghi, Complement Ther Clin Pract 2023) [8].
• Semaglutide (Ozempic, Wegovy) injections may delay gastric emptying and reduce iron absorption. A preliminary study of 51 people with type 2 diabetes showed a 13% reduction in iron absorption from ferrous fumarate after 10 weeks of semaglutide (Melis, Diabetes Obes Metab 2025) [8].

How to Read a Supplement Label

The weight of the iron compound and the weight of elemental iron differ significantly. For example [11]:

• 325 mg ferrous sulfate = approximately 65 mg elemental iron
• 325 mg ferrous fumarate = approximately 107 mg elemental iron
• 325 mg ferrous gluconate = approximately 39 mg elemental iron

Look for "elemental iron" on the Supplement Facts panel. The FDA requires that the amount of elemental iron be listed [2].

Evidence for Benefits

Iron Deficiency Anemia

Iron supplementation is the standard treatment for iron deficiency anemia (IDA). The evidence base for this is extensive and well-established [2][8][9].

Dosing for IDA correction: Traditionally, 100–200 mg elemental iron daily in divided doses was recommended. However, in September 2021, the British Society of Gastroenterology recommended only 50–100 mg of iron once daily on an empty stomach (Snook, Gut 2021) [8]. The reason: a single 60 mg dose of iron increases hepcidin levels, which blocks absorption from subsequent doses the same day. Total iron absorption from a single 60 mg daily dose was similar to that from two 60 mg doses taken the same day [8].

High-dose is no better than once-daily: A study of approximately 72,000 older adults in the U.S. with iron deficiency anemia showed that taking 325 mg ferrous sulfate several times daily was no better at increasing hemoglobin and ferritin over 3–6 months than taking 325 mg once daily or every other day (Patel, JAMA Netw Open 2024) [8].

Daily vs. every-other-day dosing: A study of 52 people with IDA found that 60 mg iron (ferrous sulfate) daily produced higher iron levels by 12 weeks than every-other-day dosing. Those taking iron daily had slightly greater hemoglobin increases, and 75% were no longer anemic versus 59% in the every-other-day group, though these differences were not statistically significant (Lin, Br J Haematol 2023) [8]. A 14-day study in young women with iron deficiency found that 60 mg iron daily was modestly better than double the dose every other day in increasing hemoglobin in red blood cells, with no increase in side effects (John, Clin Nutr 2025) [8].

Time to recovery: Approximately two months is usually required to restore hemoglobin levels. Many more months of iron supplementation may be required to fully replenish iron stores [8].

Time of day does not matter: A study of young female athletes with low ferritin found that evening iron supplementation was equally effective as morning dosing (Atwell, Eur J Sport Sci 2023) [8].

Fatigue in Non-Anemic Iron Deficiency

Even mild iron deficiency — too mild to cause anemia — may cause clinically significant fatigue. A 12-week RCT of 198 women of childbearing age (18–53 years) who were not anemic but had ferritin levels below 50 mcg/L found that 80 mg iron (ferrous sulfate, time-release pill) daily produced a 47.7% decrease in fatigue versus 28.8% with placebo — a statistically significant difference. Ferritin levels increased by 11.4 mcg/L compared with placebo. There was no significant effect on anxiety or depression (Vaucher, CMAJ 2012) [8].

Similarly, a placebo-controlled trial found that iron supplementation improved one self-reported measure of mood ("vigor"), but not anxiety or depression, among female soldiers with iron deficiency and/or iron deficiency anemia (McClung, Am J Clin Nutr 2009) [8].

Cognitive Function

Adolescent girls: Verbal learning and memory improved when adolescent girls with iron deficiency (without anemia) were treated with 130 mg elemental iron (as ferrous sulfate) twice daily for 8 weeks, which raised average ferritin from 9 to 27 ng/mL (Bruner, Lancet 1996) [8]. Adolescence appears to be a critical window for building brain iron stores. A study showed that adolescent girls (average age 13) with iron deficiency but without anemia had significantly lower iron levels in the striatum, increased volume in some striatal areas (linked to psychiatric or cognitive challenges), and smaller overall brain volume compared to those without iron deficiency. This was generally not found in boys (Fiani, JAMA Netw Open 2025) [8].

Children with mood disorders: A small open-label study found that iron supplementation (25–100 mg daily for 12 weeks) decreased certain symptoms of anxiety, depression, and irritability in iron-deficient children and adolescents diagnosed with mood or behavior disorders. Average ferritin increased from 17 to 54 ng/mL (Mikami, J Nippon Med Sch 2022) [8]. However, this study lacked a placebo control and blinding.

Broader evidence: Iron deficiency in children and adolescents is associated with cognitive achievement deficits, developmental delays, and behavioral problems. If iron deficiency is extremely severe, some of these problems can be irreversible [2][9]. The functional deficits associated with IDA include impaired cognitive function, difficulty concentrating, and psychomotor abnormalities that can lead to learning difficulties without treatment [2][9].

Pregnancy

Iron deficiency during pregnancy increases the risk of maternal and infant mortality, premature birth, low birthweight, and impaired cognitive and behavioral development in infants [2][15].

Prevalence: In the U.S., 18% of pregnant women have iron deficiency — 6.9% in the first trimester, 14.3% in the second, and 29.7% in the third [2][15].

Anemia prevention: A Cochrane Review showed that daily iron supplementation (9–90 mg) reduced the risk of anemia in pregnant women at term by 70% and of iron deficiency at term by 57%. The risk of low-birthweight newborns was 8.4% with iron vs. 10.2% without, and mean birthweight was 31 g higher with iron supplementation [15].

Does routine supplementation improve outcomes? An analysis of 17 clinical trials involving more than 24,000 pregnant women showed that routine iron supplementation (typically 30–60 mg daily) reduced the risk of iron deficiency anemia in the mother at term by 60%, but there were no statistically significant differences in rates of preterm birth, low birthweight, need for cesarean delivery, gestational diabetes, or high blood pressure (Cantor, JAMA 2024) [8].

Guidelines diverge:

• The American College of Obstetricians and Gynecologists (ACOG) recommends low-dose iron supplementation starting in the first trimester [15][16].
• The U.S. Preventive Services Task Force (USPSTF) concluded in 2024 that there is insufficient evidence to recommend routine iron screening and supplementation for asymptomatic pregnant women, though this does not apply to women with symptoms of iron deficiency or IDA [8][17].
• The CDC, WHO, and Dietary Guidelines for Americans all recommend low-dose iron supplementation (e.g., 30 mg/day) for most or all pregnant women [2][15][16].

Restless Legs Syndrome

People with low or deficient levels of iron may be more likely to suffer from restless legs syndrome (Trotti, Cochrane Database Syst Rev 2019) [8]. The American Academy of Neurology advises that 65 mg iron (from 325 mg ferrous sulfate) with 200 mg vitamin C twice daily may improve symptoms in individuals with ferritin levels below 75 ng/mL (Winkelman, Neurology 2016) [8].

This recommendation is based on a study of older men and women with ferritin levels 15–75 ng/mL given iron for three months. Restless legs severity decreased by an average of 10 points on a 0–40 scale, versus only 1 point with placebo. Ferritin increased from an average of 40 to 65 ng/mL (Wang, Sleep Med 2009) [8]. Note: restless legs syndrome can also occur in hereditary hemochromatosis (excessive iron absorption), in which case iron supplementation would not be appropriate [8].

Athletic Performance and Exercise

Iron deficiency may negatively impact athletic performance. Analysis of 1,190 competitive athletes (average age 21) in Germany found that nearly 20% had iron deficiency, with prevalence higher among females (36.9%) versus males (10.6%). Those with deficiency were approximately half as likely to reach a target VO2 max as those without deficiency (Keller, Nutrition 2024) [8].

Iron requirements increase during intense exercise due to losses through sweat, hemolysis (foot-strike), and increased gastrointestinal losses. Female and adolescent athletes are at particular risk [8][2].

Blood Donation Recovery

Among blood donors with normal hemoglobin, moderate iron supplementation (37.5 mg iron as ferrous gluconate daily) reduces the time for hemoglobin and ferritin recovery after donating a unit of blood by more than half. Those receiving iron recovered most of the hemoglobin decrease within about 30 days versus 78–158 days without iron. Ferritin returned to normal within 76 days with iron, while 67% of unsupplemented donors had not recovered iron stores after 168 days (Kiss, JAMA 2015) [8][18]. About 25–35% of regular blood donors develop iron deficiency [18].

Heart Failure

Approximately 60% of patients with chronic heart failure have iron deficiency and 17% have IDA, associated with higher risk of death [2][19]. However, a clinical trial of high-dose oral iron (150 mg as iron polysaccharide twice daily for 16 weeks) in heart failure patients did NOT improve peak oxygen uptake or exercise capacity, and only minimally improved iron levels (3% increase). A possible explanation was abnormally high hepcidin levels in these patients, reducing absorption and trapping iron in white blood cells and liver cells (Lewis, JAMA 2017) [8]. Intravenous iron has been more effective in this population [2].

Thyroid Function

Iron is required for conversion of thyroxine (T4) to triiodothyronine (T3) and for proper utilization of iodine. Iron deficiency and IDA can cause hypothyroidism, characterized by low T4/T3 and elevated TSH (Garofalo, Nutrients 2023; Zimmermann, Annu Rev Nutr 2006; Ghiya, J Endocr Soc 2019) [4][8].

ADHD in Children

A small study of iron in children (ages 5–8) with ADHD showed improvement in some symptoms comparable to that with stimulant medicines when given 80 mg iron daily as ferrous sulfate (Konofal, Pediatr Neurol 2008) [8]. This is preliminary evidence and requires confirmation in larger trials.

Premenstrual Syndrome (PMS)

A lower risk of PMS has been associated with high intake of iron (over 20 mg/day) from supplements or plant-based foods. Compared to women with the lowest intakes (median 9.4 mg daily), risk was reduced by 29% with a median intake of 21.4 mg and by 36% with 49.2 mg (Chocano-Bedoya, Am J Epidem 2013) [8].

ACE Inhibitor-Induced Cough

Iron supplements (51.2 mg daily as 256 mg ferrous sulfate) have shown promise for reducing the dry cough caused by ACE inhibitor medications (Lee, Hypertension 2001) [8].

Fibromyalgia

A study in Taiwan found that women with iron deficiency anemia had a 22% increased risk of fibromyalgia compared to women without IDA. When receiving iron supplementation, their risk of fibromyalgia was 21% lower than those not receiving iron therapy (Yao, Sci Rep 2021) [8]. The researchers recommended that all people with fibromyalgia have their blood iron levels checked.

Mental Health

Iron deficiency has been associated with an increased risk of anxiety, depression, and certain other psychiatric disorders in observational studies (Lee, BMC Psychiatry 2020; Ahmed, Brain Behav 2023) [8]. However, interventional evidence that iron supplementation improves mood beyond correcting deficiency-related fatigue is limited to small, unblinded studies.

Infants and Young Children

Iron deficiency anemia in infancy can lead to adverse cognitive and psychological effects, including delayed attention and social withdrawal, some of which may be irreversible [2][9]. IDA is also associated with higher blood lead concentrations, increasing neurotoxicity risk [2].

Evidence for supplementation: A Cochrane Review of 26 studies in 2,726 preterm and low-birthweight infants found that enteral iron supplementation (at least 1 mg/kg/day) reduces the risk of iron deficiency, though long-term effects on neurodevelopment are unclear [2]. Another Cochrane Review of 8 trials in 3,748 children showed that home fortification with micronutrient powders containing iron reduced anemia by 31% and iron deficiency by 51% [2].

Guidelines: The American Academy of Pediatrics recommends 1 mg/kg/day iron supplementation for exclusively breastfed full-term infants from age 4 months until iron-containing foods are introduced [2]. Preterm infants should receive 2 mg/kg/day from ages 1–12 months [2].

Anemia of Chronic Disease

Certain inflammatory, infectious, and neoplastic diseases can cause anemia of chronic disease (also known as anemia of inflammation), the second most common type of anemia after IDA [2]. Inflammatory cytokines upregulate hepcidin, diverting iron from circulation to storage sites and limiting its availability for red blood cell production [2]. This condition is usually mild to moderate (hemoglobin 8–9.5 g/dL) and can be difficult to diagnose because ferritin levels tend to be elevated by inflammation [2].

Treatment primarily involves addressing the underlying disease. The use of oral iron supplementation in this setting is controversial because it may increase infection risk and cause tissue damage [2]. Parenteral (IV) iron increases hemoglobin more effectively and with fewer side effects than oral iron in this population [2].

Recommended Dosing

Recommended Dietary Allowances (RDAs)

From the Institute of Medicine / National Academies [2][6]:

Age Group	Male (mg/day)	Female (mg/day)	Pregnancy	Lactation
0–6 months	0.27*	0.27*	--	--
7–12 months	11	11	--	--
1–3 years	7	7	--	--
4–8 years	10	10	--	--
9–13 years	8	8	--	--
14–18 years	11	15	27	10
19–50 years	8	18	27	9
51+ years	8	8	--	--

*Adequate Intake (AI). Source: Institute of Medicine [2][6].

Vegetarians need 1.8 times the RDA because nonheme iron from plant-based foods has lower bioavailability than heme iron from animal products [2][6].

Tolerable Upper Intake Level (UL)

The UL for iron is 45 mg/day for adults (ages 14+) and 40 mg/day for children under 13 [2][6]. These limits are based on gastrointestinal side effects from iron salt supplements. However, physicians frequently prescribe doses exceeding the UL when treating iron deficiency anemia [2].

Note: these limits may be too low for people with hereditary hemochromatosis, who are at unique risk for accumulating harmful levels of iron [8].

Practical Dosing by Indication

• Treating iron deficiency anemia: 50–100 mg elemental iron once daily on an empty stomach (British Society of Gastroenterology, Snook, Gut 2021) [8]. Previously, 100–200 mg daily in divided doses was standard, but evidence now shows a single daily dose is equally effective due to hepcidin-mediated absorption blockade from second doses [8].
• Preventing anemia during pregnancy: 27 mg/day (the RDA) to 30–60 mg/day, depending on guidelines [2][15][16].
• After bariatric surgery: 45–60 mg/day after Roux-en-Y gastric bypass or sleeve gastrectomy (Mechanick, Endocr Pract 2019). However, a 2-year study found that even 100 mg daily post-surgery did not reliably prevent hemoglobin decline, suggesting currently recommended regimens may be insufficient (Lener, Nutr Metab Cardiovasc Dis 2023) [8].
• Fatigue in non-anemic women with low ferritin: 80 mg daily from a time-release ferrous sulfate pill (Vaucher, CMAJ 2012) [8].
• Blood donation recovery: 37.5 mg daily as ferrous gluconate (Kiss, JAMA 2015) [8][18].
• Restless legs syndrome (ferritin <75 ng/mL): 65 mg iron (from 325 mg ferrous sulfate) with 200 mg vitamin C, twice daily (Winkelman, Neurology 2016) [8].
• Cognitive function in iron-deficient adolescent girls: 130 mg elemental iron (as ferrous sulfate) twice daily for 8 weeks (Bruner, Lancet 1996) [8].
• ADHD in children (ages 5–8): 80 mg iron daily as ferrous sulfate (Konofal, Pediatr Neurol 2008) [8].
• For healthy non-vegetarian adult men and postmenopausal women: Iron supplementation is generally NOT recommended unless blood tests show deficiency. These groups are at low risk of deficiency and at potential risk of iron excess [8].

When to Take Iron

• Preferably on an empty stomach for maximum absorption. However, taking with food is acceptable when GI side effects are a problem [8].
• Avoid taking with green tea, coffee (can reduce absorption by up to 50%), though coffee consumed at least one hour before supplementation does not appear to affect absorption [8].
• Time of day does not appear to matter — morning and evening dosing are equally effective (Atwell, Eur J Sport Sci 2023) [8].
• Avoid taking within 24 hours of an MRI, as iron can cause imaging artifacts (Green, Cureus 2018; Cook, J Magn Reson Imaging 2006) [8].
• Iron can temporarily increase serum iron and transferrin saturation — avoid supplements and iron-rich foods at least 5–9 hours before blood tests checking iron status [8].
• Iron supplementation generally does not need to be stopped before ferritin testing alone — it does not cause falsely elevated or decreased ferritin levels [8].

Safety and Side Effects

Common Side Effects

The most common side effect of iron supplementation is gastrointestinal distress, including [2][8][11]:

• Constipation — the most frequently cited complaint
• Nausea and stomach upset — especially when taken on an empty stomach
• Diarrhea
• Abdominal cramping
• Black or dark-colored stools (harmless discoloration from unabsorbed iron)

These side effects are more common with ferrous salts (sulfate, fumarate, gluconate) at higher doses and when taken without food. Chelated forms (bisglycinate), heme iron polypeptides, carbonyl iron, and polysaccharide-iron complexes generally cause fewer GI side effects [11].

Strategies to Reduce GI Side Effects

• Switch from ferrous sulfate to a better-tolerated form (bisglycinate, carbonyl iron, or liquid formulation) [8]
• Take with food (reduces absorption modestly but improves tolerance) [8]
• Use once-daily dosing rather than multiple daily doses [8]
• Consider every-other-day dosing if daily dosing is intolerable [8]
• Increase dietary fiber intake and use stool softeners for constipation [8]

Iron-Pill Gastropathy

Prolonged use of iron pills — particularly ferrous sulfate — can occasionally damage the stomach lining, causing gastritis, erosions, ulcers, and gastrointestinal bleeding (iron-pill induced gastropathy). This can paradoxically worsen anemia (Sunkara, Gastroenterology Res 2017) [8]. Reported cases include:

• Two men (ages 72 and 81) and a woman (age 90) taking 325 mg ferrous sulfate 1–2 times daily, all of whom had symptoms resolve after switching to intravenous iron (Comba, ACG Annual Meeting 2019) [8].
• Two 14-year-old girls taking ferrous sulfate on an empty stomach for 2–6 weeks. In one case, switching to liquid iron resolved symptoms (Mauney, J Pediatr Gastroenterol Nutr 2025; Melit, Medicine 2017) [8].

Gastric siderosis (excess iron deposition in the stomach lining) may also occur, appearing as brown discoloration on biopsy, and can increase oxidative stress and clotting risk in the stomach (Tun, ACG Case Rep J 2022) [8].

Strategies to reduce risk include switching to liquid formulations, avoiding ferrous sulfate pills, taking iron with food, and using every-other-day dosing (Mauney, J Pediatr Gastroenterol Nutr 2025) [8].

Esophageal Injury

Although rare, iron pills can become lodged in the esophagus and cause damage, including ulceration and iron deposits. This risk is higher in people with swallowing disorders or esophageal narrowing from reflux (Stoleru, ACG Case Rep J 2025) [8].

Aspiration Risk

If iron pills are accidentally inhaled, they can cause chemical burns and serious injuries to the trachea, bronchia, and lungs due to the acidity of ferrous salts. Complications may develop days to weeks after the pill is expelled (Caterino, Respir Med Case Rep 2023) [8].

Long-Term Risks of Excessive Iron

Observational studies have found associations between mildly excessive stored iron and several chronic diseases [2][8]:

• Cardiovascular disease: Higher stored iron levels have been associated with modestly higher risk of stroke, particularly cardioembolic stroke, possibly due to iron triggering clot formation (Gill, Stroke 2018) [8]. However, among U.S. women, stroke risk actually decreased with iron intake up to 20 mg daily, with no further decrease at higher intakes. No association was found in men (Xu, Int J Environ Health Res 2021) [8].
• Cancer: Observational associations exist but do not prove causation [8].
• Type 2 diabetes: Similarly based on observational data [8].
• Mortality in postmenopausal women: Iron supplement use was associated with a 3.9% increased risk of death over 22 years, with risk increasing as dosage increased, particularly at very high doses (50 mg to several hundred mg/day) (Mursu, Arch Int Med 2011) [8].
• Cancer recurrence during chemotherapy: Use of iron supplements before or during breast cancer chemotherapy was associated with a 91% higher risk of cancer recurrence (Ambrosone, J Clin Oncol 2020) [8].

Pregnancy-Specific Risks

While pregnant women are generally encouraged to take iron supplements, supplementation beyond the recommended amount may increase complications. One study found that non-iron-deficient breastfed infants do not benefit from iron supplements and might experience decreased health if given supplementation [8].

Iron and Carotenoid Absorption

High doses of ferrous iron (but not ferric iron) may decrease absorption of carotenoids such as beta-carotene, lycopene, and astaxanthin from foods and supplements. This is due to reactions between carotenoids and divalent mineral ions (Corte-Real, Food Chem 2016; Biehler, J Nutr 2011) [8]. Take carotenoid supplements at a different time of day than iron.

Iron Poisoning

Very high single doses of elemental iron can be life-threatening [2][8]:

• 20 mg/kg body weight (~1,365 mg for a 150-lb person): Can cause corrosive necrosis of the intestine, fluid and blood loss, shock, and organ failure.
• 60 mg/kg (~4,090 mg for a 150-lb person): Can lead to multisystem organ failure, coma, seizures, and death.

Iron poisoning is most common in young children (ages 3 and younger) who ingest adult iron supplements. Between 1983 and 2000, at least 43 U.S. children died from iron supplement ingestion [2]. Supplements containing 30 mg or more of iron (except carbonyl iron) are required to carry a warning label, and those containing 250 mg or more per container must be in child-resistant packaging [2].

Hereditary Hemochromatosis

Hemochromatosis is a genetic condition (HFE gene mutation) causing excessive iron absorption and accumulation. About 1 in 10 Whites carry the most common mutation (C282Y), but only 4.4 per 1,000 are homozygous and develop the disease [2]. Without treatment (periodic phlebotomy), signs of iron toxicity develop by the 30s, including liver cirrhosis, hepatocellular carcinoma, heart disease, and impaired pancreatic function [2]. The American Association for the Study of Liver Diseases recommends that hemochromatosis patients avoid iron and vitamin C supplements [2][20].

People with hemochromatosis should also be cautious of red iron oxide (used as a supplement colorant), though absorption from this source is very low (about 0.01%). Taking it with vitamin C can increase absorption 5–50 fold, but this still results in only about 0.025 mg from a typical 5 mg dose — likely inconsequential for healthy people but worth noting for those with hemochromatosis [8].

Drug Interactions

Iron supplements interact with numerous medications. These interactions generally involve either iron reducing drug absorption (via chelation) or medications affecting iron absorption or levels.

Iron Reduces Drug Absorption

These medications should be separated from iron supplements [2][8][14]:

Drug	Separation Time	Notes
Levothyroxine (Synthroid, Levoxyl)	4+ hours	Iron reduces levothyroxine absorption. Liquid and softgel formulations may be less affected than tablets (Liu, Ther Clin Risk Manag 2023; Gatta, Front Endocrinol 2022) [8][14].
Levodopa (Sinemet, Stalevo)	2+ hours	Iron reduces levodopa bioavailability via chelation, diminishing effectiveness for Parkinson's disease [2][14].
Quinolone antibiotics (ciprofloxacin, levofloxacin)	2h before or 6h after	Forms insoluble chelation complexes [8].
Tetracycline antibiotics (doxycycline, minocycline)	1h before or 2h after	Forms insoluble complexes [8].
Penicillamine	2+ hours	Iron reduces absorption [8].
Carbamazepine	2+ hours	Iron may lower carbamazepine concentrations by ~30% (Ahn, Epilepsy Res 2019) [8].
ACE inhibitors	2+ hours	Iron may reduce absorption (possible interaction) [8].

Drugs That Affect Iron Absorption or Levels

Drug Class	Effect on Iron	Clinical Notes
Proton pump inhibitors (PPIs)	Reduce absorption	Reduce gastric acid needed for nonheme iron absorption. Long-term use (up to 10 years) does not cause depletion in those with normal stores, but patients with existing iron deficiency may have suboptimal responses to supplementation [2][14].
Low-dose aspirin	Increase loss	Daily 100 mg aspirin for 5 years increased anemia risk by 20% in adults aged ~74, regardless of whether major bleeding occurred (McQuilten, Ann Intern Med 2023) [8].
Semaglutide (Ozempic, Wegovy)	May reduce absorption	Delays gastric emptying; preliminary data showed 13% reduction in iron absorption (Melis, Diabetes Obes Metab 2025) [8].

Calcium and Iron

Calcium may interfere with the absorption of both heme and nonheme iron [2][6]. For this reason, experts suggest taking calcium and iron supplements at different times of the day [2].

Zinc and Iron

Supplements containing 25 mg or more of iron may reduce zinc absorption and plasma zinc concentrations [2]. High-dose zinc can also interfere with iron regulation [2].

Biotin and Ferritin Testing

High doses of biotin may interfere with certain blood ferritin assays, potentially producing inaccurate results [8].

Dietary Sources

Food	Serving	Iron (mg)	% DV (18 mg)	Type
Fortified breakfast cereal	1 serving	18	100%	Nonheme
Oysters, eastern, cooked	3 oz	8	44%	Heme + Nonheme
White beans, canned	1 cup	8	44%	Nonheme
Beef liver, pan fried	3 oz	5	28%	Heme + Nonheme
Lentils, boiled	1/2 cup	3	17%	Nonheme
Spinach, boiled	1/2 cup	3	17%	Nonheme
Tofu, firm	1/2 cup	3	17%	Nonheme
Dark chocolate (45–69% cacao)	1 oz	2	11%	Nonheme
Kidney beans, canned	1/2 cup	2	11%	Nonheme
Sardines, canned	3 oz	2	11%	Heme + Nonheme
Chickpeas, boiled	1/2 cup	2	11%	Nonheme
Beef, braised	3 oz	2	11%	Heme + Nonheme
Baked potato (flesh and skin)	1 medium	2	11%	Nonheme
Cashew nuts	1 oz	2	11%	Nonheme

Source: USDA FoodData Central [2][21].

Practical Notes on Dietary Iron

• Heme iron (from animal sources) is more bioavailable than nonheme iron and is less affected by dietary inhibitors. However, it represents only 10–15% of total iron intake in typical Western diets [2][6].
• Vitamin C-rich foods enhance nonheme iron absorption. Pair iron-rich plant foods with citrus fruits, bell peppers, tomatoes, or broccoli [2][6].
• Phytates in whole grains and legumes bind nonheme iron and reduce absorption. Soaking, sprouting, or fermenting reduces phytate content [2][6].
• Tea and coffee significantly inhibit nonheme iron absorption due to polyphenol content. Avoid consuming these within an hour of iron-rich meals or supplements [6][8].
• Cooking in cast iron cookware can contribute some iron to food, especially when cooking acidic foods. However, a review found mixed evidence (only about half of studies showed benefit), and iron cookware should not replace dietary changes or supplementation for preventing deficiency (Sharma, Nepal J Epidemiol 2021) [8].
• Flour fortification: In the United States, Canada, and many other countries, wheat and other flours are fortified with iron, making bread, cereal, and grain products important sources [2].
• Breast milk contains highly bioavailable iron but in amounts insufficient for infants older than 4–6 months, at which point iron-fortified complementary foods or formula become necessary [2].
• The average Western diet provides 10–20 mg of iron per day [8]. Most non-vegetarian men and postmenopausal women meet their requirements from food alone. Premenopausal women, adolescent girls, and pregnant women are most likely to fall short [2].

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