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Iron Supplements: Who Needs Them, When to Take Them and Side Effects

Iron supplements help people who are low in iron, so get a ferritin blood test first; in men and after menopause, low iron needs checks for bleeding in the gut. In menstruating women, daily iron made anemia about 60% less likely in pooled trials (grade A). Coffee, tea and meals cut absorption in small studies (grade C). Every-other-day dosing showed no clear difference from daily (grade C). Keep iron away from children: an overdose can kill.

Published Updated How we grade evidenceEditorial policy

Iron is the mineral your body uses to make hemoglobin, which carries oxygen in red blood cells (NIH ODS). Too little stored iron can lead to anemia. This guide covers adults; pregnancy has its own guide.

The short version

  • Daily iron made anemia less likely in menstruating women, in a Cochrane review (anemia result: 10 trials, 3,273 women). Grade A (Low 2016).
  • Every other day showed no clear difference from daily iron in hemoglobin (very low certainty); one trial found fewer stomach side effects. Grade C (Dhanvijay 2025; von Siebenthal 2023 trial).
  • Coffee taken with an iron pill cut absorption by 54% in a small absorption study. Grade C (von Siebenthal 2023; Morck 1983).
  • Vitamin C with iron (the same iron tablet taken with or without 200 mg of vitamin C, in 440 adults with iron-deficiency anemia): hemoglobin rose by about the same amount either way. Grade B (Li 2020).
  • Tiredness eased a little in people with low iron but no anemia. Grade B (Houston 2018).
QuestionWhat trials foundSize of effectGrade
Daily iron in menstruating womenLess anemia, higher hemoglobinRisk ratio 0.39 for anemiaA
Every other day vs dailyNo clear difference in hemoglobin; fewer stomach effects in one trialUnclearC
Vitamin C with iron (440 adults, the same iron tablet with or without vitamin C)No real gain in hemoglobin2.00 with vs 1.84 g/dL without, at 2 weeksB
Tiredness, low iron, no anemiaLess tiredness, no change in fitness testsSmallB
Hair lossLow ferritin linked to hair loss (observational); not tested as a treatmentUnknownNo trial
Bisglycinate vs other formsFewer stomach effects in pregnancy; unclear in other adultsUnclearC

Who needs iron supplements?

People who are low in iron, which a blood test shows. A 2026 American Society of Hematology (ASH) guideline lists the main risks: heavy periods, giving blood, very hard training, vegan or vegetarian diets, coeliac disease, weight-loss (bariatric) surgery, and long-term stomach-acid drugs such as proton pump inhibitors (PPIs) (ASH 2026). On a GLP-1 medicine that cuts your appetite, see supplements on GLP-1 medicines. After weight-loss surgery, your surgical team sets the plan.

Grade A for menstruating women. Across 67 trials (8,506 women) pooled by Cochrane, daily iron made anemia at the end of the trials less likely (risk ratio 0.39; 10 trials, 3,273 women; moderate certainty) and raised hemoglobin (51 trials, 6,861 women; high certainty) (Low 2016). Our iron page and menstrual health protocol show a lower grade (C) from fewer studies; this review supports A.

Grade B for blood donors. In a trial of 215 US donors, those with low ferritin who took 37.5 mg of elemental iron a day got 80% of their hemoglobin back in 32 days, against 158 days without iron (Kiss 2015). Across 7 trials (1,647 donors), oral iron probably meant fewer donors turned away for low hemoglobin (risk ratio 0.39; moderate certainty) (Geneen 2026).

Vegetarians and athletes. Vegetarian adults had ferritin about 30 µg/L lower than meat eaters across 24 cross-sectional studies (Haider 2018), and ASH lists very hard training as a risk too.

What ferritin level means you are low?

Ferritin is a blood test for stored iron. ASH's 2026 guideline suggests these cut-offs for adults without inflammation, each a conditional recommendation on low-certainty evidence (ASH 2026):

  • Adults not menstruating or pregnant: 30 ng/mL or below, instead of the older 15 ng/mL; 50 ng/mL or below with symptoms such as tiredness, or an ongoing risk such as blood donation.
  • Menstruating adults: 30 ng/mL or below; 50 ng/mL or below with heavy periods, symptoms, upcoming surgery or plans for pregnancy.
  • Inflammation (from a long illness, for example) pushes ferritin up, so ask for transferrin saturation (TSAT) too. In adults with anemia from inflammation, a TSAT under 20% or a ferritin under 100 ng/mL may point to iron deficiency.

The guideline also covers pregnancy; your midwife or obstetric team will use it.

What is the best time to take iron supplements?

Grade C: away from meals and coffee. These are small absorption studies; none measured hemoglobin. In 34 women with low iron who took 100 mg doses of tracer-labelled iron six ways, coffee cut absorption by 54% and breakfast with coffee by 66%, while a drink with 80 mg of vitamin C raised it by 30% (500 mg did no better). Afternoon doses were absorbed 37% less, a hint that mornings may absorb better, but that gap was not statistically clear (von Siebenthal 2023). If iron upsets your stomach, taking it with food can ease that (NIH ODS).

Upper limit for adults: 45 mg of elemental iron a day, because more causes stomach upset and constipation; take more only with blood tests or your clinician's advice (NIH ODS; check your stack).

Is every-other-day iron better than daily?

Grade C: no clear difference in hemoglobin, possibly fewer side effects. A dose of 60 mg or more raises hepcidin, a hormone that blocks iron uptake, so a dose the next day was absorbed 35% to 45% less (Moretti 2015). In 150 young women given 100 mg daily or on alternate days, ferritin ended no different (43.8 against 44.8 µg/L) at the same total dose, but stomach side effects were 1.56 times as common on daily iron (von Siebenthal 2023 trial).

In people with anemia, 11 pooled trials (1,014 people) found no clear difference in hemoglobin and similar side effects overall, though a metallic taste was more common on daily iron; certainty was very low for most outcomes (Dhanvijay 2025). A US gastroenterology expert review advises iron once a day at most (DeLoughery 2024).

Should you take vitamin C with iron?

Grade B: it adds little. In 440 adults with iron-deficiency anemia in China, iron alone met the trial's test for equivalence with iron plus 200 mg of vitamin C a dose: hemoglobin rose 1.84 against 2.00 g/dL in 2 weeks (Li 2020). Ten pooled studies also found only 0.14 g/dL more, a gain their authors judged likely too small to matter. That pool says little about adding vitamin C to your own iron tablet, though. Most of its studies enrolled pregnant women or young children rather than other adults, and most set two different iron products against each other, such as an iron salt made with vitamin C (ferrous ascorbate) against plain ferrous sulfate (Deng 2024). The expert review still advises vitamin C (DeLoughery 2024). Orange juice with your pill is a fair habit; a separate vitamin C pill is optional.

Can I take iron with coffee, tea, calcium or milk?

Keep coffee and tea apart from iron. In a meal study, tea cut iron absorption by 64% and coffee by 39%, and coffee an hour after eating still lowered it (Morck 1983).

Grade B for calcium: a small effect. Pooled short absorption studies found calcium lowered iron absorption a little, and longer studies found no drop in hemoglobin, though more calcium went with lower ferritin; the authors call the evidence too mixed for firm advice (Abioye 2021). Experts still advise taking calcium and iron supplements at different times, and iron doses of 25 mg or more can lower zinc absorption (NIH ODS). With a multivitamin plus separate minerals, check your stack against the upper limits.

Can I take iron with levothyroxine, omeprazole or antibiotics?

  • Levothyroxine (thyroid hormone): iron can cut how much you absorb (NIH ODS). See can I take iron, calcium or magnesium with levothyroxine?
  • PPIs such as omeprazole, and antacids: less stomach acid means less iron absorbed (NIH ODS); PPI users had more iron-deficiency anemia across 9 studies (risk ratio 2.56) (Ali 2023). Don't stop a PPI on your own; ask whether your iron should be checked.
  • Tetracycline and quinolone antibiotics (such as doxycycline and ciprofloxacin): iron cut ciprofloxacin absorption by 57% in healthy volunteers (Lehto 1994) and seriously impaired tetracycline absorption (Neuvonen 1970).
  • Levodopa (for Parkinson's disease and restless legs): iron can lower how much you absorb (NIH ODS).

Ask your pharmacist how far apart to take each one.

What is the best form of iron?

Check the label for elemental iron, the iron inside the salt: ferrous fumarate is 33% iron by weight, ferrous sulfate 20% and ferrous gluconate 12% (NIH ODS). The US expert review found no oral form better than another and prefers ferrous sulfate as the cheapest (DeLoughery 2024).

Grade C for bisglycinate (iron bound to the amino acid glycine). In 9 pooled trials in pregnant women, it raised hemoglobin more and caused fewer stomach complaints than other forms; 4 trials in children found no difference, and 4 in other adults were not pooled (Fischer 2023 review). If you are pregnant, take iron as your midwife or obstetric team advises. In 480 Cambodian women, most with enough iron, 18 mg of bisglycinate raised ferritin less than 60 mg of ferrous sulfate (Fischer 2023 trial).

Does iron help tiredness or hair loss without anemia?

Grade B for tiredness, when iron is low. In 4 pooled trials (714 people) with low iron but normal hemoglobin, 2 of pills and 2 of iron by drip, iron eased self-reported tiredness a little in both kinds, while 9 trials found no change in fitness tests (Houston 2018). In 198 tired women with ferritin below 50 µg/L and no anemia, fatigue scores fell 47.7% on 80 mg of elemental iron a day against 28.8% on placebo over 12 weeks, with no change in depression or anxiety (Vaucher 2012). These trials enrolled people with low iron, so they don't show iron lifts energy when your iron is normal. Our fatigue page shows a lower grade (C); this pooled analysis and trial support B.

No trial for hair loss. Women with hair loss that leaves no scarring had lower ferritin than other women, in studies of 10,029 people; that is a link, not a test of iron pills (Treister-Goltzman 2022). Our hair loss page calls the iron evidence moderate, but no trial has tested iron pills for hair loss; our hair loss guide covers regrowth.

Restless legs: ask your clinician to check your ferritin. In a Cochrane review, iron by mouth or by drip eased symptoms more than placebo (7 trials, 345 people), though every trial had some unclear risk of bias; whether to use iron, and which kind, is a medical decision (Trotti 2019). See our restless legs protocol.

Does iron cause constipation or black stools?

Grade A for gut side effects. In menstruating women, iron roughly doubled hard stools or constipation (risk ratio 2.07; 8 trials, 1,036 women) and loose stools (risk ratio 2.13; 6 trials, 604 women), both at high certainty (Low 2016). In 20 trials against placebo, adults taking ferrous sulfate had more than twice the odds of stomach and gut side effects (odds ratio 2.32), and dark or black stools are common with iron pills (Tolkien 2015). For relief, see our constipation guide.

What we'd skip

  • Iron without a blood test. Benefit trials enrolled people with low iron, and in 38,772 older Iowa women, iron supplement use was linked with a 10% higher death rate (observational, so not proof of cause) (Mursu 2011).
  • Twice-a-day doses: three doses in 24 hours absorbed no more iron in total than two morning doses (Moretti 2015).
  • Iron with your morning coffee: absorption fell by more than half (von Siebenthal 2023).

Safety and interactions

  • Side effects: constipation, nausea, stomach pain, vomiting and diarrhea are the common ones (NIH ODS).
  • Upper limit for adults: 45 mg of elemental iron a day, because more causes stomach upset and constipation; take more only with blood tests or your clinician's advice (NIH ODS; check your stack). Count the iron in any multivitamin.
  • Keep iron away from children. US rules make iron supplement labels warn that accidental iron overdose is a leading cause of fatal poisoning in children under 6 (21 CFR 101.17). In one hospital series, 2 of 5 young children who swallowed their pregnant mothers' iron tablets died of liver failure (Chandran 2023).
  • Iron overload: haemochromatosis, an inherited condition, makes the body store too much iron, which can damage the liver; transferrin saturation and ferritin tests find it (EASL 2022). Skip iron unless a blood test finds you short of it.
  • Interactions: see the medicines section above.
  • IV iron is a prescription medicine for when pills don't work, can't be tolerated or can't be absorbed (DeLoughery 2024). With kidney disease, see our renal anemia protocol.

When to see a doctor

  • Emergency: a child who may have swallowed any iron (get emergency help or call poison control right away), or black, tar-like stools with dizziness, fainting or vomiting blood.
  • Soon, before starting iron: men, and women past menopause, whose iron is low for no clear reason. About a third have a cause in the gut, such as bleeding or bowel cancer, that needs tests (Snook 2021).
  • Soon: heavy periods, tiredness, finding exercise harder, a racing heart or cravings for ice, which can go with low iron (ASH 2026).

Don't stop a prescribed medicine on your own; ask the prescriber or pharmacist. If you are pregnant or breastfeeding, ask your midwife or obstetric team. If you are under 18, ask a clinician.

FAQ

What is the best time of day to take iron? Away from meals, coffee and tea (grade C, from small absorption studies). In 34 women with low iron, coffee cut absorption by 54%; one study hinted that mornings absorb better, but the gap was not statistically clear.

Should I take iron every day or every other day? Pooled trials in people with anemia found no clear difference in hemoglobin (very low certainty), and one trial in 150 young women found fewer stomach side effects on alternate days (grade C).

How long does iron take to work? In a trial of 440 adults with iron-deficiency anemia taking iron three times a day under medical care, hemoglobin rose about 2 g/dL in 2 weeks; lower doses work more slowly, and stores take longer to refill, so ask for a repeat blood test.

What ferritin level is too low? A 2026 hematology guideline suggests 30 ng/mL or below for most adults, and 50 ng/mL or below for adults with symptoms or ongoing risks such as heavy periods.

Sources

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