Iron supplement forms: sulfate, bisglycinate and gluconate

Reviewed by Emma Carter, Senior Health & Nutrition Editor

Two iron supplements can carry the same big number on the front of the bottle and give you completely different amounts of iron. Sorting that out first solves most of the confusion about which form to buy.

The short version: ferrous sulfate is the benchmark, it works, and it genuinely upsets a share of the people who take it. Ferrous bisglycinate had fewer gut effects in pregnancy trials, but evidence outside pregnancy is thin and guidelines do not name it the best form overall. Gluconate and fumarate are ferrous salts at different strengths. How often you take iron can also affect tolerance.

The number on the label is not the iron you get

An iron salt is iron bonded to something else, and only part of the tablet is iron. That part is elemental iron, and it is the only figure worth comparing between products.

Form	Elemental iron	Tablet size for 60 mg of iron
Ferrous fumarate	About 33%	180 mg
Ferrous sulfate (heptahydrate)	About 20%	300 mg
Ferrous gluconate	About 12%	500 mg

Equivalences from a 2024 review of oral iron therapy (PMC / NIH).

A 500 mg gluconate tablet and a 180 mg fumarate tablet hand you the same iron. So a bottle shouting "500 mg" is describing the salt, not the dose. Read the supplement facts panel for the words "elemental iron". Chelated products such as ferrous bisglycinate are labelled less consistently, so check the panel there too rather than assuming.

The forms, side by side

Form	What the evidence covers	Tolerability	Sensible for
Ferrous sulfate	The most studied form and the comparator in most trials	More gut side effects than placebo	A first attempt, and where cost matters
Ferrous bisglycinate	17 randomized trials, strongest in pregnancy	Fewer gut effects in the pregnancy trials	After standard salts are poorly tolerated; discuss pregnancy use with your clinician
Ferrous fumarate	Same salt chemistry as sulfate	Broadly similar to sulfate	Fewer or smaller tablets for the same iron
Ferrous gluconate	A ferrous salt with lower iron density	No proven overall tolerability advantage	When its elemental dose and tablet format suit the prescribed plan

Ferrous sulfate: it works, and the side effects are real

Pooling 43 randomized trials covering 6,831 adults, ferrous sulfate roughly doubled the odds of gastrointestinal side effects against placebo (odds ratio 2.32, 95% CI 1.74 to 3.08) and tripled them against intravenous iron (3.05, 2.07 to 4.48) (PLoS One, 2015).

In the same paper, meta-regression found no relationship between iron dose and the odds of side effects across trials. That does not prove dose never affects an individual, but it does mean that "a smaller dose is always gentler" is not a reliable rule.

The NHS lists the common effects as nausea or vomiting, stomach discomfort or heartburn, loss of appetite, constipation, diarrhea, darker poo and black stained teeth (NHS). Darker poo is expected on iron. Black, tar-like or bloody stool is not, and the NHS says to speak to a doctor or call 111 straight away.

Ferrous bisglycinate: strong evidence in one group, thin outside it

Bisglycinate is iron chelated to the amino acid glycine, sold as the gentle option and priced to match. A systematic review pooled 17 randomized trials of it against other iron supplements (Nutrition Reviews, 2023).

In pregnant women, across nine trials, bisglycinate raised hemoglobin more than the comparison supplements (standardized mean difference 0.54, 95% CI 0.15 to 0.94) and produced fewer gut side effects, with an incidence rate ratio of 0.36 (95% CI 0.17 to 0.76). That is roughly a third of the event rate.

In children, across four trials, there was no difference in hemoglobin or ferritin.

For everyone else, including non-pregnant women, only four trials existed and the reviewers did not pool them at all. They described them instead. So if you are not pregnant, the case for bisglycinate rests on results collected in a different group. That can still be the right call after sulfate has defeated you. It is not the settled question the packaging implies, and it is worth knowing before you pay two or three times as much.

Daily or alternate-day dosing

In women with depleted iron stores (ferritin 25 µg/L or below), the same fourteen 60 mg doses were absorbed better when spread over 28 days than when taken on consecutive days: cumulative fractional absorption was 21.8% versus 16.3%, and 175.3 mg versus 131.0 mg of iron was absorbed (Lancet Haematology, 2017). This was an absorption study, not proof that every patient should switch schedules.

Splitting the day's dose in two does not help either. Twice-daily dosing raised hepcidin and gave no absorption benefit over a single morning dose.

Hepcidin is part of the reason. A dose of 60 mg or more can raise it for up to 24 hours, reducing absorption from the next dose (PMC / NIH). The American Gastroenterological Association advises no more than one oral dose a day and says every-other-day dosing may be better tolerated for some people, with similar or equal absorption (AGA, 2024).

What to actually buy

• Test before you buy anything. A ferritin test, or a full iron panel if inflammation might be distorting the picture, tells you whether you need iron at all. Our guide to iron deficiency covers what the numbers mean.
• Compare elemental iron, never tablet weight.
• Start with ferrous sulfate or fumarate if cost matters. They are cheap, they work, and you will find out quickly whether you tolerate them.
• Consider another formulation if sulfate is not tolerable. Bisglycinate has encouraging pregnancy trial data, but current NHS pregnancy guidance still lists ferrous sulfate or fumarate as suitable standard preparations (NHS Greater Glasgow and Clyde). Our guide to iron in pregnancy has the dosing context.
• Use the schedule you were given. Once-daily treatment is common. Alternate-day dosing can be considered when daily tablets are not tolerated, including in the NHS pregnancy guidance above.
• Keep working on food too. Iron-rich foods will not correct a deficiency on their own, but they hold ground once you have.

The full routine, including vitamin C, tea and coffee timing, and the gap you need from thyroid medication, is in our main iron supplement guide.

When to see a doctor

Iron is not a supplement to take on a hunch. The body has no easy route to shed the excess, so taking it without a confirmed shortfall brings risk and no benefit.

See your doctor if tiredness, breathlessness or hair shedding has you reaching for iron without a test, if you have been on iron for three months with no change in how you feel, or if you cannot tolerate any form you try. Poor tolerance is a reason to review the plan, not to give up on correcting the deficiency.

Frequently asked questions

Which iron supplement is gentlest on the stomach?

Ferrous bisglycinate produced fewer gastrointestinal adverse events than comparison supplements across nine pregnancy trials. Outside pregnancy the evidence is much thinner, and the American Gastroenterological Association says no oral formulation has a proven overall advantage. It can be a reasonable switch after intolerance, not a proven first choice for everyone.

How much elemental iron is in ferrous sulfate?

About 20 percent by weight, so a 300 mg tablet of ferrous sulfate heptahydrate gives roughly 60 mg of iron. Ferrous fumarate is about 33 percent, so 180 mg gives the same 60 mg, and ferrous gluconate is about 12 percent, so it takes 500 mg. Compare the elemental iron figure, not the tablet weight.

Does a lower dose of iron cause fewer side effects?

Not reliably. Across 43 ferrous sulfate trials in 6,831 adults, meta-regression found no relationship between dose and the odds of gastrointestinal side effects. That analysis does not prove dose never matters for an individual. If side effects persist, ask about the dose, schedule or formulation rather than reducing treatment on your own.

Should I take iron every day or every other day?

Either may be used, depending on the reason for treatment and how well you tolerate it. Current clinical advice uses no more than one oral dose a day; every-other-day dosing may be better tolerated for some people with similar or equal absorption. In pregnancy, some NHS guidance uses once-daily treatment and offers alternate-day dosing when daily tablets are not tolerated. Follow the plan given by your clinician.

Is ferrous gluconate gentler than ferrous sulfate?

There is no strong head-to-head evidence that gluconate is consistently gentler. It contains less elemental iron by weight, so compare the elemental dose rather than tablet size. Tolerance varies, and a clinician or pharmacist can help you change formulation or schedule if side effects persist.

Is black poo from iron a problem?

Darker poo is common on iron and usually nothing to worry about. Black, tar-like or bloody stool is different: the NHS says to speak to a doctor or call 111 straight away, because that can signal bleeding in the gut.

References

1. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis · Tolkien Z et al, PLoS One, 2015
2. The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials · Fischer JAJ et al, Nutrition Reviews, 2023
3. Iron absorption from oral iron supplements given on consecutive versus alternate days in iron-depleted women · Stoffel NU et al, Lancet Haematology, 2017
4. Oral iron therapy: current concepts and future prospects for improving efficacy and outcomes · PMC / NIH, 2024
5. Side effects of ferrous sulfate · NHS
6. Management of iron deficiency anemia · American Gastroenterological Association, 2024
7. Prevention and management of iron deficiency anaemia in pregnancy and the puerperium · NHS Greater Glasgow and Clyde