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Iron

Iron

Best for

Rebuild iron stores when depleted

Proven strong benefit · 1–400 mg/day for 6–52 weeks · 12 meta-analyses , n=43.8k

47 papers · 5 claims · 87 outcomes scored · 5 positive

Evidence summary

Evidence summary

Proven strong benefit

Iron delivers a proven strong benefit for rebuilding depleted iron stores and improving iron-deficiency anemia, especially when ferritin or hemoglobin is low.

  • Across 18 studies (n=43,815), iron rebuilt depleted iron stores and improved iron-deficiency anemia outcomes.1
  • The evidence spans 47 papers and 87 outcomes, including fatigue, ferritin, hemoglobin, and anemia.
  • Benefits are strongest with low ferritin or anemia; normal labs leave less room for improvement.

Outcomes

What iron actually does, by outcome

Each row is one outcome with effect size, evidence base, the dose that worked in trials, and time to first effect. Magnitude tiers come from native-unit MCID where available, Cohen's d otherwise.

Rebuild iron stores when depleted Proven strong benefit

Raises circulating iron, fills reserves, and feeds iron-hungry tissues.

18 meta-analyses n=44k 1–400 mg 6–52 wk
28510680·38147281·39469104·40464050 +14 more
Prevent anemia through pregnancy Large effect, needs confirmation

Keeps red blood cell levels up when pregnancy drains them fastest.

4 meta-analyses n=23k 30–200 mg 8–24 wk #1/3
26280534·27087396·37403900·39145520
Reduce everyday fatigue Likely benefit

Less of that heavy, worn-out feeling that makes simple tasks feel like a chore.

8 meta-analyses n=986 14–10000 mg 4–12 wk #1/44
12763985·22272750·22777991·29626044 +4 more
Calm restless legs at night Likely benefit

Quiets the crawling, tingling urge to move when your body needs to be still.

2 meta-analyses n=638 1000–10000 mg 10 wk #1/3
30609006·32380660
Build more oxygen-rich red blood cells Proven modest benefit

More red cells loaded with more hemoglobin reach every muscle and organ.

24 meta-analyses n=65k 2–1000 mg 2–52 wk #1/5
22272750·22419305·22777991·24130243 +20 more

Forms & standardisation

The best-studied products are standard ferrous salts, especially ferrous sulfate, with ferrous fumarate and ferrous gluconate also showing up often in trials.1 Check the label for elemental iron, because the pill weight is not the same as the iron dose your body actually gets.1

Risk profile

Adverse events and known drug interactions

Safety events

Serious adverse events (any) severe
Serious adverse events (hospitalization or death) severe
Altered neurodevelopment / long-term neurodevelopmental impairment severe
constipation mild
abdominal pain mild
gastrointestinal discomfort mild
increased appetite mild
dark stools mild

Drug interactions

Antineoplastic Agents major decreases effect
Metformin major decreases concentration
Aspirin moderate decreases concentration

Co-studied with

Supplements that share evidence with iron

Vitaminb9 9 shared outcomes · d=0.24
Mineral 7 shared outcomes · d=0.22
Vitaminc 1 shared outcome

Frequently asked

Common questions

How long does iron take to work?

Fatigue trials usually ran 4–12 weeks, and blood markers often move over 8–16 weeks, so you judge iron by the two-month mark, not after a few days.21

Should you take iron with vitamin C?

You do not need vitamin C for iron to work, but you do want to avoid calcium, tea, and coffee around your dose because they blunt absorption.3

Why does iron upset your stomach?

Iron commonly causes constipation and belly pain because some of the dose stays in the gut and irritates it. If that happens, a smaller dose or a different salt often feels easier.1

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Sources

  1. 1. Iron-Deficiency Anemia (2015) ↑
  2. 2. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial (2003) ↑
  3. 3. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women (2015) ↑

Generated May 15, 2026

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