Ferrous Sulfate

Ferrous sulfate is the classic, cheapest and best-studied oral iron salt, and the reference form in most iron-deficiency trials and clinical guidelines. It is highly effective at correcting deficiency but is also the most likely of the common salts to cause GI upset (nausea, constipation, cramping). For iron’s full, form-agnostic evidence base — who benefits, effect sizes, pregnancy, dietary intake — see the Iron hub. Only what is specific to the sulfate form is covered here.

Absorption & Tolerability

Ferrous sulfate is the gold-standard comparator: when a study reports another salt’s “relative bioavailability,” ferrous sulfate is usually the 100% baseline it is measured against. It is well absorbed as divalent (ferrous, Fe²⁺) iron via the intestinal DMT1 transporter — its reputation rests on being effective and reliable, not on being gentle. In head-to-head clinical reviews of the two dominant oral forms, ferrous (sulfate) and ferric (iron-polymaltose) preparations achieve broadly comparable repletion, with the practical trade-off being tolerability against cost rather than a large efficacy gap ^{1Reference 1Santiago P · 2012ReviewFerrous versus ferric oral iron formulations for the treatment of iron deficiency: a clinical overview — reviewView study →}.

Its practical downside is tolerability. A systematic review and meta-analysis of 43 RCTs (6,831 adults) found ferrous sulfate roughly doubled the odds of GI side-effects versus placebo (OR 2.32, 95% CI 1.74–3.08) and tripled them versus intravenous iron (OR 3.05, 95% CI 2.07–4.48). Notably, meta-regression found no significant relationship between dose and side-effect risk — the irritation is largely intrinsic to the salt, not simply a high-dose phenomenon ^{2Reference 2Tolkien Z et al. · 2015Meta-analysisFerrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysisView study →}. Nausea, epigastric discomfort, constipation and dark stools are the usual reason people switch to a gentler chelate.

A few form-specific practical points:

• Elemental iron is ~20% in the common heptahydrate (FeSO₄·7H₂O) and higher, around 30%, in the dried or anhydrous salt — always read the elemental iron figure on the label, not the salt weight.
• Take on an empty stomach where tolerated, since food reduces absorption; accept some loss if food is needed to make it bearable.
• Vitamin C is optional, not essential. Ascorbic acid modestly boosts single-meal non-heme absorption in isotope studies, but a 3-month equivalence RCT in adults with iron-deficiency anaemia found adding 200 mg of vitamin C gave no additional rise in haemoglobin or ferritin over iron alone — routine pairing is not required for repletion ^{3Reference 3Li N et al. · 2020RCTThe efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical (equivalence) trialView study →}.
• Separate from inhibitors — tea and coffee polyphenols, calcium supplements, and antacids or PPIs all reduce absorption.
• Alternate-day, single-morning dosing improves fractional absorption and reduces side effects versus split daily dosing (see below).

What the Evidence Says

Because ferrous sulfate is the reference form, most of the general iron-deficiency evidence is effectively sulfate evidence — it is the salt the majority of efficacy trials actually used. There is little that is uniquely “sulfate” beyond its established role as the low-cost, first-line standard for treating diagnosed iron deficiency and iron-deficiency anaemia. For the full deficiency and anaemia evidence base, and the scored applications table, defer to the Iron hub — nothing here re-scores iron’s uses.

What is genuinely form-specific and decision-relevant is the dosing schedule, worked out largely with labelled ferrous sulfate:

• A single oral dose transiently raises hepcidin, which then blunts absorption of the next dose given later the same day or the following day. In iron-depleted non-anaemic women, doses of 60 mg or more acutely increased serum hepcidin and reduced absorption from twice-daily and consecutive-day regimens ^{4Reference 4Moretti D et al. · 2015Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women — comparative studyView study →}.
• Alternate-day, once-daily dosing therefore absorbs better than split or daily dosing. In iron-depleted women, alternate-day single morning doses gave higher fractional absorption than consecutive-day or twice-daily dosing ^{5Reference 5Stoffel NU et al. · 2017RCTIron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two randomised controlled trialsView study →}; the effect also held in women with established iron-deficiency anaemia, where fractional absorption on alternate days was roughly 40–50% higher than on consecutive days ^{6Reference 6Stoffel NU et al. · 2020Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic womenView study →}.
• The trade-off is favourable. A 6-month double-blind RCT (n=150) found that at equal total iron doses, alternate-day dosing produced the same ferritin repletion as daily dosing while GI side-effects were meaningfully lower on non-dosing days (side-effect prevalence ratio on iron-intake days 1.56, 95% CI 1.38–1.77) ^{7Reference 7von Siebenthal HK et al. · 2023RCTAlternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled studyView study →}.

In practice: lower the elemental dose or move to alternate days before switching salts ^{8Reference 8Stoffel NU et al. · 2020ReviewOral iron supplementation in iron-deficient women: how much and how often? — reviewView study →}.

Dosage

These are doses studied in research, not a personal recommendation. Supplement iron only for a diagnosed need, under clinician guidance.

Iron is dosed as elemental iron, not as the weight of the sulfate salt — the single most important labelling point for this form. Because ferrous sulfate is about 20% elemental iron as the common heptahydrate, a standard 325 mg tablet delivers roughly 65 mg of elemental iron.

• Dietary reference (form-agnostic, from the Iron hub): RDA 8 mg/day for adult men and post-menopausal women, 18 mg/day for menstruating women, 27 mg/day in pregnancy.
• Therapeutic repletion: clinician-directed for a diagnosed deficiency, typically 40–120 mg elemental iron/day. Recent absorption work suggests lower, less-frequent dosing may be as effective with better tolerance ^{4,5,6,7,8Reference 4Moretti D et al. · 2015Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women — comparative studyView study →Reference 5Stoffel NU et al. · 2017RCTIron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two randomised controlled trialsView study →Reference 6Stoffel NU et al. · 2020Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic womenView study →Reference 7von Siebenthal HK et al. · 2023RCTAlternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled studyView study →Reference 8Stoffel NU et al. · 2020ReviewOral iron supplementation in iron-deficient women: how much and how often? — reviewView study →}.
• Alternate-day dosing: a single morning dose on alternate days can give higher fractional absorption and fewer side-effects than divided daily dosing — a practical first move before switching to a gentler, and pricier, chelate.

See the Iron hub for full intake references, including the 45 mg/day UL.

Safety

Do not supplement iron without a diagnosed need. The safety profile is iron’s, not the sulfate ion’s — see the Iron hub for the full base. Form-specific points:

• Gastrointestinal upset is the dose-limiting effect for this form. Sulfate is the most GI-irritating common salt: nausea, constipation, cramping and dark stools, roughly doubling GI-side-effect odds versus placebo ^{2Reference 2Tolkien Z et al. · 2015Meta-analysisFerrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysisView study →}. Lower the elemental dose, move to alternate-day dosing, or take it with food to manage them.
• Acute overdose is the serious hazard. Iron overdose is genuinely dangerous and a leading cause of fatal poisoning in young children; ingested tablets can cause corrosive GI injury, shock and liver damage. Keep away from children. This underlies the moderate toxicity rating.
• Interactions. Iron binds several substances in the gut — separate it by at least 2–4 hours from tea and coffee, calcium supplements, fluoroquinolone and tetracycline antibiotics, levothyroxine, and bisphosphonates. Antacids and PPIs reduce absorption.
• Vitamin C co-dosing does not increase adverse events, but it is not needed for efficacy ^{3Reference 3Li N et al. · 2020RCTThe efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical (equivalence) trialView study →}.

Pregnancy and lactation: iron requirements rise in pregnancy (RDA 27 mg/day) and ferrous sulfate is a first-line treatment for gestational iron deficiency. Sulfate also carries the highest GI-side-effect burden of the common salts in pregnant women — a pooled OR of about 3.33 across seven RCTs ^{2Reference 2Tolkien Z et al. · 2015Meta-analysisFerrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysisView study →} — which is why lower-dose or gentler forms are sometimes preferred. Supplementation should follow assessment of iron status and clinician guidance rather than being assumed universally.

Scope disclosure. Interactions were assessed: chelating drug classes (fluoroquinolones, tetracyclines, levothyroxine, bisphosphonates) and absorption-reducers (antacids/PPIs, calcium, polyphenols). Pregnancy and lactation were assessed, with the deeper base on the hub. An Upper Limit does exist and is form-agnostic — 45 mg/day elemental iron, set for GI tolerability, and therapeutic repletion under clinician guidance can exceed it deliberately.

Related compounds / cross-links

• Hub: Iron — the full, form-agnostic evidence base, intake references, and safety.
• Sibling forms: Ferrous fumarate (~33% elemental, most iron per tablet, intermediate tolerance), Ferrous gluconate (~12% elemental, gentler, more tablets), Ferrous bisglycinate (amino-acid chelate, gentlest, premium).
• Enhancer: Vitamin C — modestly increases single-meal non-heme absorption, but is not required for repletion.

References

1. Santiago P (2012). Ferrous versus ferric oral iron formulations for the treatment of iron deficiency: a clinical overview — review. The Scientific World Journal. https://pubmed.ncbi.nlm.nih.gov/22654638/
2. Tolkien Z, Stecher L, Mander AP, Pereira DIA, Powell JJ (2015). Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. https://pubmed.ncbi.nlm.nih.gov/25700159/
3. Li N, Zhao G, Wu W, et al. (2020). The efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical (equivalence) trial. JAMA Network Open. https://pubmed.ncbi.nlm.nih.gov/33136134/
4. Moretti D, Goede JS, Zeder C, et al. (2015). Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women — comparative study. Blood. https://pubmed.ncbi.nlm.nih.gov/26289639/
5. Stoffel NU, Cercamondi CI, Brittenham G, et al. (2017). Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two randomised controlled trials. The Lancet Haematology. https://pubmed.ncbi.nlm.nih.gov/29032957/
6. Stoffel NU, Zeder C, Brittenham GM, Moretti D, Zimmermann MB (2020). Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica. https://pubmed.ncbi.nlm.nih.gov/31413088/
7. von Siebenthal HK, Moretti D, Zimmermann MB, Stoffel NU (2023). Alternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled study. EClinicalMedicine. https://pubmed.ncbi.nlm.nih.gov/38021373/
8. Stoffel NU, von Siebenthal HK, Moretti D, Zimmermann MB (2020). Oral iron supplementation in iron-deficient women: how much and how often? — review. Molecular Aspects of Medicine. https://pubmed.ncbi.nlm.nih.gov/32650997/

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Last checked: August 2026.