Iron Bisglycinate vs Ferrous Sulfate
Iron bisglycinate (also called ferrous bisglycinate) costs $0.11 to $0.32 a day in the bottles we price; ferrous sulfate is the older, cheaper standard, at $0.04 a day for a 65 mg tablet.
Prices checked · 5 products · 5 registries searched
Trials report more constipation and nausea with ferrous sulfate than with placebo (PMID 25700159). Bisglycinate was studied at lower doses: in one small pregnancy trial, 25 mg of iron as bisglycinate was compared with 50 mg as ferrous sulfate (PMID 24152889). Cost and label dose for the products we price are below. Take iron only if a blood test and your clinician say you need it.
Lowest-cost bisglycinate: NOW Foods Iron 36 mg Double Strength, $0.11 a day, 36 mg iron ($0.030 per 10 mg)
Check price →Ferrous sulfate: Nature Made Iron 65 mg, $0.04 a day, 65 mg iron ($0.006 per 10 mg); USP Verified
Check price →As an Amazon Associate we earn from qualifying purchases. Ranked by cost per 10 mg of iron, never commissions. Not medical advice.
Cost per 10 mg of iron, both forms
| Product | Form | Iron per serving | Cost per day (checked Oct 5, 2026) | Per 10 mg iron | Registry check | |
|---|---|---|---|---|---|---|
| Nature Made Iron 65 mg | Ferrous sulfate | 65 mg | $0.04 | $0.006 | USP Verified “Nature Made Iron 65 Mg Tablets” listed Informed Choice Product line “Nature Made Iron” listed | Check price → |
| NOW Foods Iron 36 mg Double Strength | Bisglycinate | 36 mg | $0.11 | $0.030 | None stated | Check price → |
| Solgar Gentle Iron (Iron Bisglycinate) 25 mg | Bisglycinate | 25 mg | $0.11 | $0.043 | None stated | Check price → |
| Thorne Iron Bisglycinate 25 mg | Bisglycinate | 25 mg | $0.27 | $0.107 | NSF Certified for Sport NSF #1205059, 60-capsule bottle listed NSF/ANSI 173 “Iron Bisglycinate” listed | Check price → |
| MegaFood Blood Builder | Bisglycinate | 26 mg | $0.32 | $0.124 | None stated | Check price → |
Doses differ (25 to 65 mg of iron per serving), so compare the per-10-mg column. Cost per day = Amazon price ÷ servings in the bottle (one serving a day). Per 10 mg iron = price ÷ (servings × mg of iron per serving) × 10. Iron per serving is the elemental iron on each label; Nature Made’s 65 mg comes from 325 mg of ferrous sulfate. We searched NSF Certified for Sport (full list read October 1, 2026), NSF/ANSI 173 (list saved August 25, 2026), Informed Choice (list read September 22, 2026), Informed Sport and USP Verified. NSF lists each product by flavor and pack, so we count an NSF listing only when it covers the product we link.
Head-to-head comparison
| Factor | Iron Bisglycinate | Ferrous Sulfate |
|---|---|---|
| Effectiveness | Compared with sulfate at a lower iron dose in one pregnancy trial (PMID 24152889) | The reference standard |
| Stomach side effects in trials | Usually fewer; clearest in pregnancy trials (PMID 36728680) | More constipation, nausea and dark stools than placebo (PMID 25700159) |
| With food and inhibitors | Less affected by calcium, tea, coffee and phytates | Absorption cut by calcium, tannins, coffee and phytates |
| Iron per pill | Printed on the label: 25, 26, 36 mg in the bottles we price | About 20% by weight: 325 mg ferrous sulfate = about 65 mg iron |
| Cost per day | $0.11 to $0.32 (4 bottles) | $0.04 (Nature Made Iron 65 mg) |
| Cost per 10 mg iron | $0.030 at the lowest | $0.006 |
Why ferrous sulfate causes side effects
Ferrous sulfate breaks apart into free iron ions in the gut. Iron that is not absorbed can irritate the gut lining, which is the likely source of the nausea, cramping and constipation. A systematic review found ferrous sulfate causes significantly more stomach side effects than placebo (Tolkien 2015, PMID 25700159). Ferrous sulfate is not a bad product: it is the cheap, effective standard, and many people take it without trouble.
How iron bisglycinate is different
Iron bisglycinate binds each iron atom to two glycine molecules. That shields the iron from food inhibitors such as calcium, tannins and phytates, and it tends to leave less free iron in the gut. The limits: the strongest tolerability evidence is in pregnant women (Fischer 2023 meta-analysis, PMID 36728680), a meta-analysis found no advantage in children, and several supporting trials were funded by iron makers.
What studies found about alternate-day dosing
How often iron is taken mattered as much as the form in these studies. One iron dose raises the hormone hepcidin for about 24 hours, and hepcidin lowers absorption of the next dose. In controlled studies, one morning dose every other day was absorbed better per milligram than daily or twice-daily doses (21.8% vs 16.3% in iron-depleted women; Stoffel 2017, PMID 29032957; Moretti 2015, PMID 26289639). The same was seen in iron-deficient anemic women (Stoffel 2020, PMID 31413088). These trials measured absorption, not symptoms. Your clinician sets the schedule that fits you.
First: do you need iron at all?
Iron is not a general wellness supplement. Whether to take it is a question for you and your clinician after a blood test. The RDA is 8 mg a day for men and 18 mg for premenopausal women; the upper limit is 45 mg a day (NIH Office of Dietary Supplements). Adult men and postmenopausal women usually do not need extra iron and can risk overload. Keep iron away from children: accidental iron overdose is a leading cause of poisoning in young kids.
Why doctors default to ferrous sulfate
Ferrous sulfate is on the WHO Essential Medicines List, costs very little and has decades of trial data. It is the usual default. Iron bisglycinate is newer, costs more per milligram of iron and is not yet in most guidelines. If you have side effects on any iron product, talk to your clinician about the options, including alternate-day dosing.
All iron forms at a glance
Ferrous sulfate and bisglycinate are the two forms compared above, but they are not the only iron salts on shelves:
| Form | Bioavailability | Side Effects | Cost Tier |
|---|---|---|---|
| Ferrous Sulfate | Moderate — the reference standard other forms are measured against | Significantly more GI side effects than placebo — constipation, nausea, dark stools (PMID: 25700159) | Very Low |
| Iron Bisglycinate (Ferrous Bisglycinate / Ferrochel) | Matches ferrous sulfate at a lower elemental dose — 25mg bisglycinate performed as well as 50mg sulfate in a pregnancy trial (PMID: 24152889) | Usually fewer GI effects than sulfate — clearest in pregnancy trials (PMID: 36728680); a meta-analysis found no advantage in children, and some supporting trials are industry-funded | Moderate |
| Ferrous Gluconate | Moderate | Generally better tolerated than sulfate, though less studied head-to-head against bisglycinate | Low |
| Ferrous Fumarate | Similar to ferrous sulfate | Similar GI profile to ferrous sulfate | Low |
| Carbonyl Iron | Lower peak absorption than the ferrous salts; absorbed slowly and steadily rather than all at once | Lower peak absorption than ferrous salts; substantially lower acute-toxicity risk | Low–Moderate |
| Item | Value (%) |
|---|---|
| Carbonyl Iron | ~100% |
| Ferrous Fumarate | ~33% |
| Ferrous Sulfate | ~20% |
| Ferrous Gluconate | ~12% |
Detailed breakdown by form
Ferrous Sulfate
Bioavailability: Moderate — the reference standard other forms are measured against · Elemental iron: ~20% (a 325mg tablet is about 65mg elemental iron) · Cost: Very Low
Dissociates into free Fe²⁺ ions in the gut. Unabsorbed iron left behind is irritating and the likely driver of the classic nausea/constipation. Listed on the WHO Essential Medicines List; widely used and low cost.
Iron Bisglycinate (Ferrous Bisglycinate / Ferrochel)
Bioavailability: Matches ferrous sulfate at a lower elemental dose — 25mg bisglycinate performed as well as 50mg sulfate in a pregnancy trial (PMID: 24152889) · Elemental iron: Labeled directly on the product — 25–36mg elemental per capsule in the products we track (no fixed weight %, chelate hydration varies) · Cost: Moderate
Iron chelated to two glycine molecules, absorbed largely as an intact chelate. That shields it from food/mineral inhibitors (calcium, tannins, phytates) that cut ferrous sulfate absorption, and tends to leave less free iron irritating the gut.
Ferrous Gluconate
Bioavailability: Moderate · Elemental iron: ~12% (a 325mg tablet is about 38mg elemental iron) · Cost: Low
A step down from ferrous sulfate in elemental strength — the lower dose per pill can mean less GI irritation, but there is less trial data on it specifically than on sulfate or bisglycinate.
Ferrous Fumarate
Bioavailability: Similar to ferrous sulfate · Elemental iron: ~33% (a 325mg tablet is about 106mg elemental iron) · Cost: Low
The highest elemental-iron fraction of the common ferrous salts, so it packs more iron per pill — but tolerability tracks with sulfate, not with bisglycinate.
Carbonyl Iron
Bioavailability: Lower peak absorption than the ferrous salts; absorbed slowly and steadily rather than all at once · Elemental iron: ~100% (pure elemental iron powder — no salt to dilute it) · Cost: Low–Moderate
Metallic iron particles that have to be oxidized in the gut before they can be absorbed — that slow release is exactly what blunts the acute-overdose risk that makes ferrous salts dangerous to young children. Less common on U.S. supplement shelves than the ferrous salts above.
For the full ranked buying guide, see best iron supplement.
Safety check · Iron
What the FDA's adverse-event file holds on Iron
Method: FDA adverse-event reports naming Iron: 1,072 of them, against a baseline drawn from 19 supplement categories. Reporting is voluntary and carries no denominator, so this counts reports, not people harmed, and cannot tell you the risk of taking Iron. How we read FAERS.
Frequently asked questions
Is iron bisglycinate the same as ferrous bisglycinate?
Yes. Iron bisglycinate, ferrous bisglycinate and iron bisglycinate chelate are names for the same thing: iron bound to two glycine molecules. Ferrochel is a branded version of it. Compare products by the milligrams of iron on the Supplement Facts panel.
Which costs less per mg of iron?
Ferrous sulfate. Nature Made Iron 65 mg costs $0.006 per 10 mg of iron. The lowest-cost bisglycinate we price, NOW Foods Iron 36 mg Double Strength, costs $0.030 per 10 mg. Doses differ (25 to 65 mg of iron per serving), so compare per 10 mg, not per pill. Prices were recorded on October 5, 2026.
Is iron bisglycinate better absorbed than ferrous sulfate?
Not by a large margin. Controlled trials do not support “2 to 3 times better absorbed” claims. In one small pregnancy trial, 25 mg of iron as bisglycinate was compared with 50 mg as ferrous sulfate and the results were similar (PMID 24152889). Bisglycinate is also less affected by food inhibitors.
Does iron bisglycinate cause fewer side effects?
Trials report more constipation and nausea with ferrous sulfate than with placebo (PMID 25700159). A 2023 meta-analysis found fewer stomach side effects with bisglycinate, mostly in pregnancy trials, and some of those trials were industry-funded (PMID 36728680). At low doses the gap narrows.
What did studies find about how often to take iron?
One dose raises the hormone hepcidin for about a day, and hepcidin lowers absorption of the next dose. In small studies, one morning dose every other day was absorbed better per milligram than daily dosing (PMID 29032957). Your clinician sets the schedule.
Do I even need an iron supplement?
Only if a blood test and your clinician say so. Men and postmenopausal women usually do not need extra iron and can risk overload. Keep iron away from children; accidental iron overdose is a leading cause of poisoning in young children.
Why do doctors still prescribe ferrous sulfate?
It is cheap and long studied. It is on the WHO Essential Medicines List and has decades of trial data. If you have side effects, talk to your clinician about the options, including alternate-day dosing.
Related guides
Sources
- Tolkien Z, et al. "Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis." PLoS One. 2015;10(2):e0117383. PMID: 25700159
- Fischer JAJ, et al. "The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of RCTs." Nutr Rev. 2023;81(8):904-920. PMID: 36728680
- Milman N, et al. "Ferrous bisglycinate 25 mg iron is as effective as ferrous sulfate 50 mg iron in the prophylaxis of iron deficiency and anemia during pregnancy in a randomized trial." J Perinat Med. 2014;42(2):197-206. PMID: 24152889
- Duque X, et al. "Effect of supplementation with ferrous sulfate or iron bis-glycinate chelate on ferritin concentration in Mexican schoolchildren: a randomized controlled trial." Nutr J. 2014;13:71. PMID: 25023784
- Stoffel NU, et al. "Iron absorption from oral iron supplements given on consecutive versus alternate days... two randomised controlled trials." Lancet Haematol. 2017;4(11):e524-e533. PMID: 29032957
- Moretti D, et al. "Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women." Blood. 2015;126(17):1981-1989. PMID: 26289639
- Stoffel NU, et al. "Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women." Haematologica. 2020;105(5):1232-1239. PMID: 31413088
- NIH Office of Dietary Supplements. "Iron: Fact Sheet for Health Professionals." ods.od.nih.gov
- NSF Certified for Sport listing #1205059 (Thorne Iron Bisglycinate, 60 capsules), opened 2026-10-01; USP Verified list (“Nature Made Iron 65 Mg Tablets”); Nature Made Iron 65 mg label, NIH Dietary Supplement Label Database #302766.
- Amazon prices recorded 2026-10-05; iron per serving from each product’s label.