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Iron

Also known as: Ferrous sulfate, Ferrous bisglycinate

Essential for carrying oxygen in the blood. Supplements help only if you're low — get tested (ferritin) first.

Common forms: Ferrous sulfate · Ferrous bisglycinate · Ferrous gluconate

Draft summary, not yet reviewed by a qualified health professional · Last updated 2026-11-03

Talk to a healthcare professional first

This information is for education only and is not medical advice, diagnosis, or treatment. Supplements can interact with medications and health conditions. Check with your doctor or pharmacist before starting anything, especially if you take medication, are pregnant or breastfeeding, or have a health condition. These statements have not been evaluated by the FDA; supplements are not intended to diagnose, treat, cure, or prevent any disease.

What the evidence says

Energy & fatigue

BModerate evidence

In non-anemic adults with iron deficiency (low ferritin), a systematic review found iron supplementation reduced self-reported fatigue. It is not expected to help people with normal iron levels.

Dose used in studies
As directed by a clinician based on blood tests (commonly 60–120 mg elemental iron every other day)
Time to effect
4–12 weeks
Most relevant for
Only people with confirmed low iron/ferritin
Studies (1)

Safety

Common side effects

  • Constipation
  • Nausea
  • Dark stools
  • Stomach pain

Avoid or ask your doctor first if

  • You haven't confirmed low iron with a blood test
  • You have hemochromatosis or another iron-overload condition

Pregnancy & breastfeeding

Likely safe at normal dietary doses — confirm with your provider.

Upper limit

45 mg/day elemental iron unless prescribed. Keep away from children — iron overdose is dangerous.

Medication interactions

  • moderateThyroid medication: Reduces levothyroxine absorption — separate by at least 4 hours.
  • moderateCertain antibiotics: Reduces antibiotic absorption — separate by at least 2 hours.

This list covers common medication classes only and is not exhaustive.

Buying tips

Look for third-party testing seals (USP Verified, NSF Certified, or Informed Choice) and match the form and dose used in studies.

Where to buy (affiliate)

Further reading