This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician or qualified healthcare provider before starting any supplement regimen.
Iron deficiency is the most common nutritional deficiency in the world, affecting an estimated 1.6 billion people. If your ferritin is low, supplementation is often necessary — but the form of iron you choose can mean the difference between smooth recovery and weeks of nausea, constipation, and GI distress. Iron bisglycinate and ferrous sulfate are the two most widely used forms, and they differ dramatically in tolerability.
Here is what the evidence says.
Iron Bisglycinate (Chelated Iron): Overview
Iron bisglycinate is elemental iron chelated to two molecules of the amino acid glycine. This chelation protects the iron from interacting with other compounds in the gut and allows it to be absorbed through amino acid transport pathways rather than the standard iron transport mechanism.
Key Benefits
- Dramatically Fewer Side Effects: A 2014 study in Acta Haematologica found that iron bisglycinate produced 90% fewer GI side effects compared to ferrous sulfate at equivalent elemental iron doses.
- High Bioavailability: Despite lower elemental iron per tablet, the chelated form is absorbed approximately 3–4 times more efficiently than ferrous sulfate.
- Does Not Interfere with Other Nutrients: Unlike ferrous sulfate, bisglycinate does not inhibit the absorption of calcium, zinc, or other minerals when taken together.
Typical Dosage
20–25 mg of elemental iron per day (equivalent in effect to 50–60 mg of ferrous sulfate due to superior absorption).
Ferrous Sulfate: Overview
Ferrous sulfate is the oldest, cheapest, and most commonly prescribed iron supplement. It delivers a high dose of elemental iron per tablet (65 mg per 325 mg tablet) and is the form used in most clinical iron deficiency trials.
Key Benefits
- Most-Studied Form: The majority of iron supplementation research — including WHO protocols — uses ferrous sulfate as the standard.
- High Elemental Iron Per Dose: Each 325 mg tablet provides approximately 65 mg of elemental iron, making it efficient for rapid repletion.
- Extremely Affordable: Ferrous sulfate costs as little as $2–$5 for a month's supply, making it accessible worldwide.
Typical Dosage
65 mg of elemental iron (one 325 mg tablet) taken 1–3 times daily, preferably on an empty stomach with vitamin C to enhance absorption.
Head-to-Head Comparison
| Factor | Iron Bisglycinate | Ferrous Sulfate | |--------|------------------|----------------| | Bioavailability | 3–4x higher than ferrous sulfate | Standard (the benchmark) | | Elemental iron per dose | 20–25 mg typical | 65 mg per 325 mg tablet | | GI side effects | Minimal (90% fewer in studies) | Common — nausea, constipation, black stools | | Can take with food | Yes (minimal absorption reduction) | Ideally on empty stomach (food reduces absorption 40–60%) | | Interaction with other minerals | Minimal | Competes with calcium, zinc, copper | | Staining/taste | None | Metallic taste, stains teeth (liquid form) | | Cost (30-day supply) | $10–$20 | $2–$5 | | Best For | Sensitive stomachs, long-term supplementation, mild-to-moderate deficiency | Severe deficiency requiring rapid repletion, budget constraints |
The Verdict
Choose Iron Bisglycinate if you have a sensitive stomach, have tried ferrous sulfate and could not tolerate the side effects, need to take iron alongside other minerals (like in a multivitamin), or have mild-to-moderate iron deficiency where you can afford a slower but more comfortable repletion.
Choose Ferrous Sulfate if you have severe iron deficiency anemia and need the highest possible dose of elemental iron quickly, your doctor has specifically prescribed it, or cost is a significant concern. Pair it with 200 mg of vitamin C and take it on an empty stomach to maximize absorption. If side effects are intolerable, try taking it every other day — a 2017 study in The Lancet found that alternate-day dosing actually improved fractional absorption and reduced side effects.
For most people who need ongoing iron supplementation — including menstruating women and endurance athletes — iron bisglycinate provides a much better experience with comparable effectiveness at the right dose.
Track Your Choice
Iron levels (ferritin, hemoglobin) should be monitored with blood tests during supplementation. Use Supplement Tracker to log your daily iron intake, record your lab results over time, and track energy levels as your stores recover.
