If you menstruate, daily iron trades constipation for less anaemia
Both halves come from the same 67 trials in 8,506 menstruating women. Haemoglobin rose and fatigue eased; loose stools and constipation each roughly doubled.
What the study found
Daily iron cuts anaemia risk by 61% and doubles constipation
Both halves come from the same 67 trials in 8,506 menstruating women. Iron raised haemoglobin by 5.3 grams per litre, improved exercise performance and eased fatigue; loose stools and constipation each roughly doubled. Benefit and harm carry the same high-quality rating.
The caveat · 10 of 67 trials at low risk of bias
Only 10 of the 67 trials were judged at low risk of bias. The harm figures rest on far fewer women than the benefit ones — 521 to 604 against 6,861 for haemoglobin — so they are much less precise. The review's composite measure of any side effect at all was not significant, even though the specific ones were.