Does iron help with tiredness?

What the trials found

A 2018 review pooled 18 trials with 1,170 adults who had low iron stores but normal hemoglobin, a state called iron deficiency without anemia.^[1] In 4 trials with 714 people, iron reduced self-reported fatigue (standardized mean difference 0.38).^[1] The trials in the review gave iron by mouth, by injection into a muscle or into a vein, at many doses. Of the 4 fatigue trials, 2 gave iron by mouth and 2 into a vein.^[1]

One trial was done in France. It included 198 women aged 18 to 53 who felt tired with no clear cause, had ferritin below 50 µg/L and hemoglobin above 12.0 g/dL.^[3] They took 80 mg of elemental iron as ferrous sulfate or a placebo every day for 12 weeks.^[3] The fatigue score fell by 47.7% with iron and by 28.8% with placebo.^[3] Quality of life, depression and anxiety did not improve more with iron than with placebo.^[3]

A Cochrane review of 67 trials in menstruating women also concluded that daily iron appears to reduce fatigue.^[4] A 2025 review of 18 studies in children, teenagers and menstruating adults without anemia found a similar effect on fatigue in randomized trials (d = 0.34).^[2]

Feeling less tired is not the same as measured fitness

The 2018 review found no change in measured physical capacity.^[1] Maximal oxygen uptake (VO2max), the most oxygen the body can use during hard exercise, did not differ in 9 trials with 235 people. Timed exercise tests did not differ either.^[1] People reported less tiredness, but the tests did not show better performance.^[1]

Why it may work

1. Iron is part of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the tissues.^[5]
2. Iron is also part of myoglobin, a protein that supplies oxygen to muscle.^[5]
3. When iron runs low, the stores fall first. Hemoglobin falls only at the last stage, iron deficiency anemia.^[5]
4. Even without anemia, low iron has been linked to fatigue, and it may affect how the brain works.^[2]

Who was studied

In the 2025 review, 1,176 of the 1,408 people were menstruating adults.^[2] The French trial included only women.^[3] When the 2025 review left out the people with iron deficiency, the effects of iron disappeared.^[2]

Who should be careful

Tiredness has many causes. Ferritin, a blood test of the body's iron stores, is the most efficient test for iron deficiency.^[5] In men and in women after menopause, iron deficiency anemia most often comes from blood loss in the gut or from poor absorption. A doctor looks for the source of the blood loss, such as colon cancer.^[6]

In menstruating women, iron about doubled the risk of constipation and of loose stools compared with control.^[4] People with hemochromatosis, a genetic disease that makes the body store too much iron, are advised to avoid iron supplements.^[5]

What is still unknown

The fatigue result of the 2018 review rests on 4 trials.^[1] That review included only people with low iron, so it cannot say whether iron helps people with normal iron stores.^[1] The authors of the French trial suggest checking blood markers after 6 weeks of treatment.^[3]