IronMineral

• Iron deficiency anemiaLikely helps

  Once iron deficiency anemia is diagnosed, oral iron is standard first-line therapy — NIH's Office of Dietary Supplements and MedlinePlus both describe it that way [1][2].
  
  This is the use the RDA and the whole supplement category exist for. Nothing else on this page has support this settled. ^[1]

• Anemia risk in pregnancyLikely helps

  A Cochrane Review cited by NIH ODS found daily supplementation at 9–90 mg iron lowered the risk of anemia at term by 70% and of iron deficiency at term by 57% [1].
  
  Infants of supplemented mothers weighed 31 g more on average, and low birthweight fell from 10.2% to 8.4% [1]. ^[1]

• Routine supplementation in every pregnancyPossibly helps

  The two US bodies disagree, and we are not picking a winner for you.
  
  ACOG recommends low-dose supplementation from the first trimester with screening at set intervals [1]. The US Preventive Services Task Force says the evidence is insufficient to weigh benefits against harms in asymptomatic pregnant women [1].
  
  The USPSTF position explicitly does not cover women who are symptomatic, severely malnourished, or have conditions that raise iron needs [1]. ^[1]

• Iron deficiency in infants and young childrenLikely helps

  A Cochrane Review of 26 studies in 2,726 preterm or low-birthweight infants found enteral iron at 1 mg/kg/day or more lowered iron deficiency risk [1].
  
  Long-term effects on neurodevelopment and growth stayed unclear in that same review [1].
  
  A second Cochrane Review pooled 8 trials in 3,748 children under 2 in low-income countries [1]. Micronutrient powders carrying 12.5–30 mg elemental iron cut anemia rates by 31% and iron deficiency by 51%, with no effect on growth [1].
  
  Worth flagging: CDC, the American Academy of Pediatrics, WHO, and USPSTF do not agree on which infants and children should be screened or routinely supplemented [1]. ^[1]

• Anemia of chronic diseasePossibly helps

  Inflammatory, infectious, and malignant conditions disrupt iron handling through the hormone hepcidin, and the main approach is addressing the underlying disease [1].
  
  Using iron supplements for this specifically is controversial — possible increases in infection and cardiovascular events, and only a few small trials behind it [1].
  
  In chronic kidney disease, oral ferrous sulfate at 130 mg/day elemental iron slowed hemoglobin decline over a year: 0.13 g/dL lost versus 0.46 g/dL on placebo [1].
  
  In cancer-related anemia, oral iron plus an erythropoiesis-stimulating agent raised hemoglobin 2.4 g/dL over 24 weeks, more than oral iron alone [1].
  
  Intravenous iron raises hemoglobin further, with fewer side effects than oral, in this population [1]. ^[1]

• Athletic and physical performancePossibly helps

  Two lines of evidence point different directions, so this stays open.
  
  A Journal of Nutrition review cites a study where iron maintained ventilatory threshold and improved energetic efficiency in iron-deficient, nonanemic athletes [15].
  
  A broader BMJ Open systematic review of RCTs in iron-depleted nonanemic adults found reduced subjective fatigue but insufficient evidence of better objective exercise capacity [13].
  
  That review's subgroup data were underpowered, so whether athletes respond differently from non-athletes could not be settled [13]. ^[13]

• Fatigue and cognition without anemiaPossibly helps

  Low iron with normal hemoglobin is a real, separately studied state, and the trials in it are small.
  
  An 8-week trial in 81 non-anemic iron-deficient adolescent girls with ferritin at or under 12 mcg/L found better verbal learning and memory scores than placebo, p<0.02 [11].
  
  In women aged 18–35, those who were iron-sufficient outperformed iron-deficient peers at baseline; women whose ferritin rose over 16 weeks gained on attention, memory, and learning tasks [22].
  
  A 2012 CMAJ trial in nonanemic menstruating women with ferritin under 50 mcg/L found 12 weeks of iron cut fatigue almost 50% from baseline, 19% more than placebo [12]. ^[11]

• Mood and cognitive scores in non-anemic peoplePossibly helps

  A 2025 meta-analysis in Neuroscience & Biobehavioral Reviews pooled 18 studies and 1,408 participants — 1,176 menstruating adults and 204 children or adolescents [14].
  
  Iron supplementation improved anxiety, fatigue, physical wellbeing, and scores on intelligence and short-term-memory tests, with depressive symptoms improving in a subset [14].
  
  The authors add the caveat that matters most here. Benefits may be smaller in people without confirmed iron deficiency [14].
  
  So none of this transfers to someone whose iron is already normal — that generalization is exactly what the pooled evidence does not support [11][12][14][22]. ^[14]

• Depletion runs in three stages: storage iron and ferritin fall first, then transferrin saturation, and only last does hemoglobin drop into anemia [1]
• Fatigue, weakness, and difficulty concentrating — the earliest and least specific signs [1]
• Pale or sallow skin, especially the face, lips, and inner eyelids; harder to see on darker skin [8][9]
• Shortness of breath, dizziness or lightheadedness, chest pain, and a rapid or pounding heartbeat [8][9]
• Headaches, particularly on exertion, plus cold intolerance [8][9]
• Pica — craving non-food items such as ice, dirt, or clay. The ice-specific form is called pagophagia [8][9]
• Koilonychia: spoon-shaped, concave nails. Iron deficiency anemia is its most common cause, but hemochromatosis and solvent exposure produce it too, so it is not specific on its own [10]
• A sore or smooth tongue (atrophic glossitis), hair thinning, brittle nails, tinnitus, altered taste, and itching [9]
• Painful mouth-corner ulcers and difficulty swallowing in more severe or longstanding cases [9]
• Restless legs syndrome, which NHS inform classifies as secondary RLS when iron deficiency is behind it [9]
• Impaired immune function, exercise and work capacity, and body-temperature regulation [1]
• In infants and children, psychomotor and cognitive abnormalities that may not resolve once the deficiency is corrected [1]
• Serum ferritin below 30 mcg/L suggests iron deficiency; below 10 mcg/L points to iron deficiency anemia specifically [1]
• Inflammation raises ferritin and can mask a real deficiency, so a normal ferritin during infection is not reassuring [1]
• Hemoglobin under 12 g/dL from age 10 up, or under 11 g/dL in children under 10, suggests IDA. Normal hematocrit runs roughly 41–50% in men and 36–44% in women [1]
• Highest-risk groups: pregnancy, infants and young children, heavy menstrual bleeding, frequent blood donation, GI disorders or GI surgery, several cancers, and chronic heart failure [1]
• Menorrhagia affects at least 10% of menstruating women and accounts for an estimated 33–41% of IDA cases in women of reproductive age [1]
• 25–35% of regular blood donors develop iron deficiency. Donors giving 2 to 3 times a year were over 5 times as likely as first-timers to have depleted stores [1]
• About 60% of chronic heart failure patients have iron deficiency and 17% have IDA. Up to 60% of colon cancer patients are deficient at diagnosis, and 29–46% across other cancer types [1]^[1]

• Ferrous sulfateThe default, cheapest, most-studied salt.

  Elemental content

  20% elemental iron by weight [1]

  Absorption

  Under 20% of a conventional ferrous-salt dose is typically taken up in the duodenum; the rest passes unabsorbed into the colon [3].
  
  That low ceiling is why therapeutic regimens run 150–300 mg/day of elemental iron despite the gut irritation [3].

  Best for

  The default, cheapest, most-studied salt. Also the one most trials mean when they say oral iron [1].

• Ferrous gluconateA smaller iron load per tablet.

  Elemental content

  12% elemental iron by weight — the lowest of the common salts [1]

  Absorption

  The same ferrous-salt absorption profile as sulfate. Solubility, not the anion, is what drives uptake, which is why ferrous forms beat ferric ones [1].

  Best for

  A smaller iron load per tablet. You never have to do the conversion maths — the Supplement Facts panel lists elemental iron directly [1].

• Ferrous fumarateReaching a high elemental dose in fewer tablets.

  Elemental content

  33% elemental iron by weight, the highest of the three common salts [1]

  Absorption

  Ferrous (Fe2+) salts are more bioavailable than ferric (Fe3+) forms such as ferric citrate and ferric sulfate, because they dissolve more readily [1].

  Best for

  Reaching a high elemental dose in fewer tablets. The flip side is more elemental iron per swallow, and gut side effects scale with that [1].

• Ferrous bisglycinateA sensitive stomach, with the caveat that the tolerability evidence is strongest in pregnant women specifically .

  Elemental content

  An amino-acid chelate — one ferrous ion bound by two glycine molecules, not a simple salt [3]

  Absorption

  A meta-analysis of 17 RCTs found 4–20 weeks of ferrous bisglycinate gave higher hemoglobin in pregnant women than other iron supplements, SMD 0.54 g/dL [3].
  
  Gastrointestinal adverse events across 7 trials in pregnant women ran at an incidence rate ratio of 0.36, roughly a 64% lower rate [3].
  
  In 4 trials of 456 children, neither hemoglobin nor ferritin differed significantly, so the advantage is established in pregnancy and not much beyond it [3].

  Best for

  A sensitive stomach, with the caveat that the tolerability evidence is strongest in pregnant women specifically [3].

• Heme iron polypeptides, carbonyl iron, polysaccharide-iron complexesPeople who could not tolerate a standard salt.

  Elemental content

  Varies by product — the Supplement Facts panel states elemental iron directly [1]

  Absorption

  NIH ODS reports these cause fewer gastrointestinal side effects than standard ferrous and ferric salts [1].
  
  The fact sheet gives no comparative absorption figures for them, so read that as a tolerability claim, not an absorption one [1].

  Best for

  People who could not tolerate a standard salt. Doses of 45 mg/day elemental iron or more are where nausea and constipation typically start [1].

• Liquid drops versus tabletsChildren and anyone who struggles with pills.

  Elemental content

  Both come in the same salts — the difference is the delivery vehicle, not the chemistry [19]

  Absorption

  WebMD's consumer guidance states plainly that one does not work any better than the other [19].
  
  Choose on swallowing ability and convenience, not on absorption [19].

  Best for

  Children and anyone who struggles with pills. Liquid stains teeth on contact — dilute it, use a straw or a dropper aimed at the back of the mouth, and brush with baking soda if it marks [2].

• Gastrointestinal upset

  At 45 mg/day of elemental iron or more, nausea and constipation become common [1].
  
  That threshold is also what the adult UL is built on — the dose of iron salts where gut side effects start, not where organ damage does [1]. ^[1]

• Iron overdose in children

  This is the most dangerous thing about a bottle of iron, and it has nothing to do with the person it was bought for.
  
  At least 43 US children died after swallowing high-dose iron supplements between 1983 and 2000 [1].
  
  The FDA requires a specific child-poisoning warning on solid-dose iron products, and federal rules require child-resistant packaging on containers holding 250 mg or more of elemental iron [1].
  
  If a child swallows an iron pill, call a poison control centre immediately [2]. ^[1]

• Acute overdose thresholds in adults

  Single intakes above roughly 20 mg/kg body weight — about 1,365 mg for a 150-lb adult — can cause corrosive necrosis of the intestine, fluid and blood loss, shock, and organ damage [1].
  
  Around 60 mg/kg, about 4,090 mg for the same adult, the documented outcomes are multisystem organ failure, coma, convulsions, and death [1].
  
  Risk is higher when the dose is taken without food [1]. ^[1]

• Hemochromatosis

  Hereditary iron overload is a flat contraindication to iron supplements [1].
  
  About 1 in 10 White people carry the common HFE C282Y mutation, though only about 4.4 per 1,000 are homozygous and actually have the disease [1].
  
  Without periodic phlebotomy or chelation, signs of iron toxicity typically appear by the 30s: liver cirrhosis, hepatocellular carcinoma, heart disease, impaired pancreatic function [1].
  
  The American Association for the Study of Liver Diseases advises those affected to avoid both iron and vitamin C supplements, since vitamin C raises iron absorption [1]. ^[1]

• Levothyroxine

  Taking iron alongside levothyroxine (Synthroid, Levoxyl, Tirosint) can cause a clinically significant drop in the thyroid drug's effect in some patients [1].
  
  Product labels advise a gap of at least 4 hours between the two [1]. ^[1]

• Levodopa

  Iron supplements reduce levodopa absorption in healthy people, probably by chelation, and US drug labels warn they may reduce its clinical effectiveness [1].
  
  Levodopa is used for Parkinson's disease and restless legs syndrome, including in Sinemet and Stalevo [1]. ^[1]

• Fluoroquinolone and tetracycline antibiotics

  Iron chelates ciprofloxacin, levofloxacin, doxycycline, and their relatives in the gut, cutting how much of the antibiotic reaches the bloodstream [18].
  
  Human studies cited in a hospital pharmacy analysis put the drop in ciprofloxacin bioavailability at more than 50% [18].
  
  National prescribing guidance says keep iron at least 2 hours away from these antibiotics, before or after [18].
  
  A UK hospital audit still found the two given within 2 hours of each other in 19.3% of relevant prescriptions — 810 of 4,067 [18]. ^[18]

• Proton pump inhibitors

  Gastric acid helps nonheme iron dissolve, so acid suppressors like omeprazole and lansoprazole can cut absorption [1].
  
  Up to 10 years of PPI use was not linked to iron depletion or anemia in people with normal stores [1].
  
  People who are already iron deficient and on a PPI, though, can respond poorly to supplementation [1]. ^[1]

• Calcium, milk, and antacids

  This one is genuinely unsettled, and we are flagging it rather than resolving it.
  
  Calcium may reduce absorption of both nonheme and heme iron, which is unusual — most inhibitors only touch nonheme [1]. NIH's own review says the effect has not been definitively established [1].
  
  Both NIH ODS and MedlinePlus still advise separating them by at least 2 hours, or taking calcium and iron at different times of day [1][2]. ^[1]

• Zinc

  Supplements delivering 25 mg of iron or more can reduce zinc absorption and lower plasma zinc concentrations [1]. ^[1]

• Bisphosphonates

  Iron taken at the same time as alendronate or risedronate can reduce how much of the osteoporosis drug is absorbed, and with it its effect [20]. ^[20]

Two forms, absorbed very differently. Heme iron comes from meat, seafood and poultry, and is largely indifferent to what else is on the plate [1].

Nonheme iron — the only kind in plants and fortified foods — is not [1].

Heme accounts for just 10–15% of total iron intake in Western diets, despite being the better-absorbed form [1].

Overall bioavailability runs about 14–18% from mixed diets with meat, seafood, and vitamin C, versus 5–12% from vegetarian diets [1].

Per serving, per USDA data: fortified breakfast cereal at 100% DV, 18 mg; cooked eastern oysters, 3 oz, 8 mg [1].

Also around 8 mg: canned white beans, 1 cup. Pan-fried beef liver, 3 oz, comes in at 5 mg [1].

About 3 mg each: boiled lentils, boiled spinach, or firm tofu, half a cup [1].

About 2 mg each: dark chocolate (1 oz, 45–69% cacao), canned kidney beans, canned sardines, boiled chickpeas, stewed tomatoes, braised beef, a medium baked potato, oil-roasted cashews [1].

About 1 mg each: green peas, roasted chicken, enriched white rice, bread, raisins, whole-wheat spaghetti, canned tuna, turkey, pistachios, broccoli, a hard-boiled egg, brown rice [1].

Essentially none: cheddar, cantaloupe, mushrooms, cottage cheese, milk [1].

Enhancers and inhibitors matter for the nonheme half. Vitamin C is the best-documented enhancer, and meat, poultry, and seafood also raise nonheme absorption when eaten in the same meal [1].

Phytates in grains and legumes, and polyphenols in tea, coffee, and some vegetables, pull the other way [1].

Spinach is the classic trap: iron-rich by content, poorly absorbed, because the same leaves carry inhibiting polyphenols [1].

In a typical Western mixed diet these push-and-pull effects have only a modest overall impact on most people's iron status [1].

Roughly half of US dietary iron arrives via bread, cereal, and grain products, because flour is fortified in the US, Canada, and many other countries [1].

Breast milk iron is highly bioavailable but runs short in quantity past about 4–6 months, so standard infant formula is fortified to 12 mg/L [1].

There is no evidence-backed best hour of the day. No controlled trial in this research compared morning against evening dosing for a clinical outcome [1][2].

What is settled is the trade-off. Iron absorbs best on an empty stomach, which is also when it most often causes cramps, nausea, or diarrhoea [1][2].

WebMD's guidance is concrete: 1 hour before eating or 2 hours after, and if that makes you nauseated, take it with a small amount of food instead [19].

Pair it with vitamin C. Adding vitamin C to breakfast alone nearly doubled daily iron absorption in a classic controlled study; splitting the same total across every meal raised it more than three-fold [21].

Ascorbic acid works by forming a soluble chelate with ferric iron at stomach pH that stays soluble in the more alkaline duodenum, and the effect scales with the dose present [17].

Keep 2 hours between iron and milk, calcium supplements, or antacids, and skip high-fibre foods and caffeine at dosing time [1][2][19].

The newer finding is about spacing, not clock time. An oral dose of 60 mg or more spikes hepcidin, the hormone gating iron absorption, peaking near 24 hours and settling by about 48 [6][7].

That spike blocks the next day's dose, which is why alternate-day dosing raises fractional absorption per dose by roughly 35–50% in isotope-tracer studies [6][7].

Intermittent dosing — one, two, or three non-consecutive days a week — carried a relative risk of 0.41 for any gastrointestinal side effect versus daily, across six trials and 1,166 women [5].

⚠ Conflict, flagged not resolved: alternate-day dosing is not simply better absorbed overall. A 2023 eClinicalMedicine RCT found it did not raise ferritin more than daily dosing, and total iron absorbed per unit time was still lower [5].

Halving the frequency outweighs the per-dose gain. A 2023 Scientific Reports RCT reached the same directional conclusion [4][5].

That trial did find less iron deficiency at 6 months and significantly fewer gastrointestinal side effects [5].

Read it as a trade: slower total repletion for better tolerability. Its authors caution against it when rapid repletion is clinically needed [5].