What is the best form of iron?

4 forms compared by what actually reaches your bloodstream. Absorption ranges from 20% to 90% — a difference that matters more than the milligrams on the label.

4 forms

90 best absorption %

55 average %

Ranked by absorption

Higher means more of the dose reaches you

Iron Bisglycinate 90 Heme Iron 85 Iron Fumarate 25 Iron Sulfate 20

Short answer

Iron Bisglycinate absorbs best — 90%.

Gentle high-absorption iron. Typical dose 18-65 mg elemental, about $30 a month. The cheapest option is Iron Sulfate at $8, but its absorption score is only 20 of 100.

See Iron Bisglycinate

Every form side by side

Form	Absorbed	Elemental	Cost / 100 mg delivered	Per month	Dose
Iron Bisglycinate	90%	27.4%	$9.77	$30	18-65 mg elemental
Heme Iron	85%	9.1%	$215.47	$120	18-30 mg
Iron Fumarate	25%	32.9%	$7.48	$18	65-130 mg
Iron Sulfate	20%	20.1%	$5.01	$8	65-200 mg

What the label does not tell you

Iron Fumarate looks like the strongest option on paper — it is 32.9% elemental iron, the highest here. But its absorption score is 25 of 100, so a 500 mg capsule delivers about 41 mg to your bloodstream.

Elemental percentage is how much of the compound is the mineral itself — the rest is the carrier the label counts as weight.
Absorption is how much of that mineral crosses into your blood.
Cost per 100 mg delivered multiplies both. It compares forms against each other, using the absorption score rather than a measured percentage — see how we derive it. Cheapest here: Iron Sulfate at $5.01.
Absorption is also dose-dependent: fractional uptake falls as the dose rises — roughly 65% at low intakes down to 11% at high ones.

See every formula we use →

What not to take with iron

Same rules for every form — the interaction belongs to the mineral, not the carrier

Calcium Inhibits iron absorption major

Zinc High Dose Competes with iron for absorption moderate

Calcium Magnesium Zinc Can reduce iron absorption when taken together moderate

Calcium Zinc Magnesium These minerals can reduce iron absorption moderate

Separating the two by two to four hours removes most of these: they compete for the same transporters in the gut, not in the bloodstream. Splitting the day is what solves a conflict — dropping one of the two is rarely necessary.

Medication interactions

6 documented

Tetracycline Antibiotics

Iron reduces antibiotic absorption · Space doses 2-4 hours apart

Levothyroxine

Iron reduces thyroid hormone absorption · Take iron 4+ hours away from thyroid medication

Deferoxamine

Iron chelator may still affect heme iron · Avoid iron supplements with chelation therapy

Tetracycline Quinolone Antibiotics

Iron significantly reduces antibiotic absorption · Space doses 2-4 hours apart

Proton Pump Inhibitors

Acid blockers reduce iron absorption · Chelated iron may overcome this interaction

Carbidopa Levodopa

May reduce Parkinson's medication effectiveness · Space doses 2+ hours apart

Prescription medicine is your doctor's call, not ours — this is what the records list.

How much is too much

Upper limit for adults: 45 mg a day, per NIH ODS.

Signs of overshooting it: acute: nausea, vomiting, diarrhea, chronic: liver damage and cirrhosis, heart problems and diabetes, joint pain and arthritis.

340 reports to the FDA name iron as the suspect product.

Most reported: vomiting (33), dyspnoea (31), diarrhoea (28), nausea (24). Serious outcomes: 84% of them.

Counted from FDA CAERS by parsing each report's product list, not by keyword — a report where the product was merely present does not count.

Head to head

6 comparisons

Iron Fumarate vs Iron Sulfate 25% · 20% Iron Bisglycinate vs Iron Sulfate 90% · 20% Heme Iron vs Iron Sulfate 85% · 20% Iron Bisglycinate vs Iron Fumarate 90% · 25% Heme Iron vs Iron Fumarate 85% · 25% Heme Iron vs Iron Bisglycinate 85% · 90%

How to read this

Absorption is the share of the dose that reaches your bloodstream — the number that decides whether the milligrams on the label mean anything.
A cheaper form at half the absorption is not cheaper per unit delivered.
Safety here reflects tolerance at normal doses, not toxicity risk.
Evidence A means the form itself was studied, not just the parent compound.

Compiled by Denis, who maintains this database and holds no medical qualification. Absorption figures and cost-per-absorbed values are derived by published formulas from cited sources — see the arithmetic.

Common questions

What is the best form of iron?

Iron Bisglycinate — it scores 90 of 100 on absorption, the highest of the 4 forms here. Gentle high-absorption iron. Typical dose 18-65 mg elemental.

Which form of iron is worst absorbed?

Iron Sulfate at 20%. It is usually the cheapest option, which is exactly why it ends up in budget products.

Is the expensive form of iron worth it?

Compare per unit delivered, not per bottle. Iron Sulfate costs $8 a month at 20% absorption, Iron Bisglycinate costs $30 at 90%.

How much iron should you take?

Doses differ by form: Iron Bisglycinate 18-65 mg elemental; Heme Iron 18-30 mg; Iron Fumarate 65-130 mg; Iron Sulfate 65-200 mg.