Iron Supplements: Types, Dosage, Side Effects and How to Take Them

Iron deficiency is the most common nutrient deficiency in the world, and in the US it’s especially common in women with heavy periods, pregnant women, young children, frequent blood donors and people with certain digestive conditions. Iron supplements work well to correct it, but they’re also easy to get wrong: the label numbers are confusing, side effects put many people off, and taking iron you don’t need can cause harm.

This guide covers who needs iron, the main types, how to read the dose, the best way to take it and the safety rules. If you’d rather start with food, see our list of iron-rich foods.

Who needs an iron supplement?

Iron helps make hemoglobin, the protein in red blood cells that carries oxygen. When your iron stores run low, you may feel tired, short of breath, dizzy or cold, and you may notice pale skin, brittle nails, headaches or cravings to chew ice. These symptoms are common to many conditions, so don’t guess: ask your doctor for a blood test. A complete blood count and a ferritin test, which measures stored iron, are the usual starting points.

People most likely to need iron include:

• Women with heavy periods, who lose more iron each month than diet easily replaces.
• Pregnant women, whose needs rise to 27 mg a day. Most prenatal vitamins contain iron, and the CDC and many obstetricians recommend it.
• Infants and young children, especially babies born early or fed mainly cow’s milk. Only use iron drops your pediatrician recommends.
• Frequent blood donors.
• People with celiac disease, inflammatory bowel disease or weight-loss surgery, which reduce iron absorption.
• People with heart failure or chronic kidney disease, under medical supervision.
• Vegetarians and vegans, who need about 1.8 times more iron because plant iron is absorbed less well. Many meet that with food.

Low iron is a sign, not a diagnosis. In men and postmenopausal women, it can be a sign of blood loss from the gut, so your doctor may look for a cause before or while treating it.

Types of iron supplements

Percentages from the NIH Office of Dietary Supplements iron fact sheet. Check your label for the exact elemental iron per tablet.

Limited evidence Iron bisglycinate is popular because it’s often easier on the stomach. A 2023 meta-analysis found that in pregnant women, bisglycinate raised hemoglobin slightly more and caused fewer digestive side effects than standard iron salts. The analysis was industry-funded and the trials were small, so the advantage isn’t certain. If ferrous sulfate works for you without side effects, there’s no need to switch to a more expensive form.

Elemental iron: reading the label

The number on the front of the bottle is often the weight of the whole compound, not the iron in it. A “325 mg ferrous sulfate” tablet contains only 65 mg of elemental iron, which is the amount that counts. US Supplement Facts panels list elemental iron, so check there.

Doses for treating deficiency are much higher than the RDA. Doctors commonly prescribe 60 to 120 mg of elemental iron a day or every other day for adults, depending on how low your levels are. A multivitamin with 18 mg is usually enough only to maintain normal levels. Our guide to reading a supplement label explains the panel in more detail.

How to take iron for best absorption

Moderate evidence Iron absorption is controlled by a hormone called hepcidin, which rises for about a day after a dose of iron and blocks absorption of the next one.

• Take it on an empty stomach, about an hour before a meal, for best absorption. If that causes nausea, take it with a small meal; you’ll absorb a little less, but it’s better than skipping doses.
• Avoid blockers: tea, coffee, milk, calcium supplements, antacids and high-fiber cereal reduce absorption. Leave at least 1 to 2 hours between them and your iron.
• Vitamin C is optional: it increases absorption of plant iron in meals, but a 2020 trial in adults with iron-deficiency anemia found that adding vitamin C to iron tablets didn’t improve hemoglobin recovery.
• Be patient: energy often improves within a few weeks and hemoglobin within 4 to 8 weeks, but rebuilding your iron stores usually takes about 3 months. Your doctor will retest to decide when to stop.

Side effects and how to reduce them

Digestive side effects are the main reason people stop taking iron. Common ones include constipation, nausea, stomach cramps, heartburn and diarrhea. Black or very dark stools are normal with iron and harmless, though black, tarry stools with stomach pain should be checked by a doctor.

• Lower the dose or switch to alternate days.
• Take it with food or at bedtime.
• Try another form, such as iron bisglycinate.
• For constipation, drink more water and eat more fiber; see our guide to high-fiber foods.

If you still can’t tolerate oral iron, or it isn’t working, your doctor may suggest an iron infusion.

Interactions

• Levothyroxine: iron reduces absorption of thyroid medicine. Take them at least 4 hours apart.
• Some antibiotics: tetracyclines and quinolones such as ciprofloxacin bind to iron. Separate them by several hours as your pharmacist advises.
• Levodopa: iron may reduce absorption of this Parkinson’s medicine.
• Acid reducers: proton pump inhibitors and H2 blockers lower stomach acid, which can reduce iron absorption over time.
• Calcium and zinc supplements: high doses compete with iron. Take them at a different time of day. Our zinc guide covers this in more detail.

Safety warnings

How to choose a product

• Match the dose to the plan: buy the elemental iron dose your doctor recommends, not the biggest number on the shelf.
• Start simple: generic ferrous sulfate is inexpensive and effective. Switch to bisglycinate if side effects are a problem.
• Look for testing seals: USP Verified or NSF Certified confirms the label is accurate.
• Skip the extras: added herbs and “energy” blends add cost but not proven benefit. A combination with vitamin C is fine but not necessary.
• Liquid options: liquid iron can stain teeth, so drink it through a straw and rinse your mouth afterwards.

Related on Pure Nutrition Facts: our guide to whether you need a multivitamin.

Iron supplements FAQ