The Sunlight Experiment

Supplement Monograph

Ferrous Sulfate

MineralIronIron-Form

The classic, cheapest and best-studied oral iron salt — the reference form against which every other iron salt is compared; highly effective for deficiency but the most likely to cause gastrointestinal side effects.

Work in progress Updated July 2026

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; this page covers only what is specific to the sulfate form.

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 — its reputation is built on being effective and reliable, not on being gentle. Its practical downside is tolerability: dose-related GI side effects (nausea, epigastric discomfort, constipation, dark stools) are common and 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 (~30%) in the dried/anhydrous salt — always check the elemental iron figure on the label rather than the salt weight.
  • Take on an empty stomach (or with a small amount of food if it is too harsh, accepting some loss of absorption).
  • Pair with vitamin C (ascorbic acid) to modestly boost non-heme absorption.
  • Separate from inhibitors — tea and coffee polyphenols, calcium supplements, and antacids/PPIs all reduce absorption.
  • Alternate-day dosing may improve fractional absorption and reduce side effects versus twice-daily dosing — see the hub.

(Detailed form-specific research pending.)

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 — this page does not re-score iron’s uses. (Form-specific evidence review pending.)

Dosage

Iron is dosed as elemental iron, not as the weight of the sulfate salt — the single most important labeling point for this form. Because ferrous sulfate is ~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 (adult men, post-menopausal women), 18 mg/day (menstruating women), 27 mg/day (pregnancy).
  • Therapeutic repletion: clinician-directed for a diagnosed deficiency, typically in the range of 40–120 mg elemental iron/day.
  • Alternate-day dosing: a single morning dose on alternate days can give higher fractional absorption and fewer side effects than divided daily dosing — lower the elemental dose or move to alternate days before switching salts.

General guide, not a personal recommendation. Supplement iron only for a diagnosed need, under clinician guidance. See the Iron hub for full intake references (RDA and 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.

  • Gastrointestinal. Sulfate is the most likely of the common salts to cause nausea, constipation, cramping and dark stools; these are dose-related. Lower the elemental dose, dose on alternate days, or take with food to manage them.
  • Acute overdose — the serious hazard. Iron overdose is genuinely dangerous and a leading cause of fatal poisoning in young children; ingested iron tablets can cause corrosive GI injury, shock and liver damage. Keep away from children. This is the basis for the moderate toxicity rating.
  • Interactions. Iron binds several substances in the gut — separate it from tea/coffee, calcium supplements, and certain antibiotics (fluoroquinolones, tetracyclines) as well as levothyroxine and bisphosphonates, by at least 2–4 hours. Antacids/PPIs reduce absorption.
  • Pregnancy. Iron requirements rise in pregnancy (RDA 27 mg/day) and ferrous sulfate is a standard treatment for deficiency, but supplementation should follow assessment of iron status and clinician guidance rather than being assumed universally.

(Form-specific safety review pending.)

Related compounds / cross-links

General overview — full cited research pending.