Short answer: Ferrous sulfate is the standard iron salt; across 20 trials it more than doubled the odds of stomach side effects versus placebo.1 Ferrous bisglycinate is iron bound to glycine. In a review of 17 trials, it gave higher hemoglobin and fewer stomach complaints than other iron forms in pregnant women, with no difference in children.2 Most of that evidence comes from pregnancy.
| Ferrous bisglycinate | Ferrous sulfate | |
|---|---|---|
| What it is | An amino acid iron chelate2 | An iron salt; ferrous and ferric salts are the most common supplement forms3 |
| Stomach side effects | Fewer than other iron supplements in pregnant women (rate ratio 0.36)2 | Odds 2.32 times placebo across 20 trials in 3,168 adults1 |
| Hemoglobin | Higher than other iron forms in pregnant women; no difference in children2 | The usual comparison in those trials2 |
| Head-to-head pregnancy trial | 25 mg of iron a day: not inferior for preventing iron deficiency, with fewer stomach complaints4 | 50 mg of iron a day in the same trial4 |
| Black stools in that trial group | 8%5 | 31%5 |
| Shared NIH notes | Adult upper limit 45 mg a day; can interact with levodopa and levothyroxine3 | Same3 |
A 2023 review pooled 17 randomized trials that compared ferrous bisglycinate with other iron supplements for at least 4 weeks. In 9 pregnancy trials, bisglycinate gave higher hemoglobin and about a third as many stomach side effects. In 4 trials in children there was no difference, and the 4 trials in men and non-pregnant women were reviewed without pooling.2
In a Danish trial, 80 pregnant women took 25 mg of iron a day as bisglycinate or 50 mg as sulfate from mid-pregnancy to delivery. Iron deficiency was equally rare in both groups, and stomach complaints were fewer with bisglycinate.4 A later report from the same group found black stools in 8% on bisglycinate and 31% on sulfate.5
A 2015 meta-analysis of 43 trials found ferrous sulfate raised the odds of stomach side effects 2.32-fold versus placebo, and the effect did not track the dose.1 In iron-depleted women given ferrous sulfate, 60 mg on alternate days was absorbed better overall than the same dose every day, and splitting a dose in two did not help.6 NIH advises taking calcium and iron at different times, and notes that acid-blocking drugs can reduce iron absorption.3
In pregnancy trials, bisglycinate was gentler on the stomach and worked at a lower iron dose. Outside pregnancy there are few comparisons. This is general information, not personal advice. More detail: the Iron guide.