Do women who work out need to take iron?

Short answer

Only when you are low. Iron deficiency is common in women who exercise, and when deficient women took iron, endurance and fatigue improved. The studies showing endurance and fatigue benefits were in deficient women, and iron pills more than doubled stomach side effects.

Do women who work out need to take iron?
'Exercise + periods → low iron → iron pills are a must' — does this formula hold?
A common saying goes that women who exercise lose iron through periods and sweat, so they must take iron pills. We checked whether everyone needs it and who actually benefits.
How common is low iron?
Women had 4.35 times the risk
Of 1,190 people screened before sport, 19.7% were low in iron, and 64.5% of those were women. Even after accounting for age and other factors, women had 4.35 times the risk. A 2026 analysis put iron deficiency at 15–35% of women who exercise.
What changes when you are low?
Endurance dropped by 3–4%
In the 1,190-person screening, people low in iron had a peak oxygen uptake (a fitness score) of 43.4, lower than 45.6 in those who were not. An analysis that pooled 669 women found iron deficiency cut endurance performance by 3–4%.
What happens when deficient women take iron?
Endurance rose 2–20% and fatigue fell
In the analysis of 669 iron-deficient women, taking 100 mg of iron a day for up to 56 days improved endurance performance by 2–20%. When 198 women with stored iron below 50 took 80 mg a day for 12 weeks, fatigue scores fell 47.7%, more than with a fake pill (28.8%).
Does it help strength too?
Short bursts of power did not beat a fake pill
In the same analysis, short all-out power changed by –5% to +9% after iron, with no difference from the fake-pill group. The clear effect was on endurance. And all 669 women in that analysis were deficient, with stored iron below 40.
So why not just take it anyway?
Stomach side effects were 2.3 times higher
An analysis of 43 trials with 6,831 people found the most common iron pill (ferrous sulfate) caused 2.3 times more stomach side effects than a fake pill. Another analysis of 8,506 women found those on iron had 2.07 times the constipation and 2.13 times the diarrhea.
If you take it, how?
Every other day beat every day
150 women low in iron took the same total amount either daily or every other day. The daily group had 1.56 times the stomach side effects, and after 6 months 11.4% were still deficient versus 3.0% in the every-other-day group. In a 40-person experiment, every other day also gave higher absorption: 21.8% versus 16.3% daily.
Summary: confirm you are low, then take it
It is for deficient women, not everyone
Low iron is common in women who exercise, and being low cut endurance. The studies where endurance rose 2–20% and fatigue fell were in deficient women, and iron pills added stomach side effects. Confirming deficiency with a stored iron (ferritin) test and then taking it every other day works.