What's the best supplement for low iron? A complete guide

November 21, 2025 •

3 min read

According to the American Society of Hematology, many people with iron deficiency may need between 150-200 mg of elemental iron daily, which is significantly more than found in multivitamins. To effectively address a deficiency, knowing what's the best supplement for low iron is key to maximizing absorption while minimizing discomfort.

Quick Summary

This article provides a comprehensive guide to choosing the best iron supplement by comparing common types, detailing how to boost absorption with dietary strategies, and offering tips to manage common side effects like constipation and nausea.

Key Points

Get a Diagnosis: Before starting, confirm an iron deficiency with blood tests from a doctor, as high doses of iron can be toxic.
Choose the Right Form: Ferrous bisglycinate is often recommended for its high absorption and minimal side effects, while ferrous sulfate is a cheaper alternative with a higher risk of stomach upset.
Maximize Absorption: Take iron supplements with Vitamin C on an empty stomach to boost absorption, and avoid taking them at the same time as calcium, dairy, or caffeine.
Manage Side Effects: To minimize constipation and nausea, try a gentler iron form, increase fiber and water, or take the supplement with a small amount of food.
Follow Duration Guidelines: Expect to take iron for several months, even after symptoms improve, to fully replenish your body's iron stores.
Consider Alternate-Day Dosing: For some, taking iron every other day can improve absorption and reduce side effects.

Understanding Your Iron Deficiency

Before considering any supplement, it is crucial to confirm an iron deficiency with a healthcare provider, as taking excessive iron can be toxic. A doctor will use blood tests, such as a complete blood count (CBC) and a serum ferritin test, to diagnose the condition and its severity.

Symptoms of low iron, which can range from mild to severe, often include:

Extreme fatigue and weakness
Pale skin
Chest pain, rapid heartbeat, or shortness of breath
Headaches and dizziness
Cold hands and feet
Brittle nails
Restless legs syndrome

These symptoms worsen as the deficiency becomes more severe. It's important to find the root cause, which can range from inadequate dietary intake to blood loss from menstruation or internal issues.

The Main Types of Oral Iron Supplements

Oral iron comes in several forms, each with a different elemental iron content and absorption profile.

Common types include:

Ferrous Salts (Ferrous Sulfate, Gluconate, Fumarate): These are common and cost-effective. Ferrous sulfate is often a first-line treatment but is prone to gastrointestinal side effects. Ferrous fumarate and gluconate may be better tolerated.
Ferrous Bisglycinate (Chelated Iron): Iron is bound to amino acids in this form, making it easier on the digestive system with less risk of constipation. Studies show it can be as effective as ferrous salts.
Polysaccharide Iron Complex: Often marketed as 'gentle iron', research suggests it might not be as effective in increasing hemoglobin levels as ferrous sulfate.

Ferrous vs. Ferric: What's the Difference?

Iron exists in ferrous (Fe²⁺) and ferric (Fe³⁺) states. Ferrous iron is more soluble and readily absorbed, making supplements with ferrous forms more bioavailable. Ferric iron is less absorbed; non-heme iron from plants is in this state. Ferric citrate and ferric maltol are sometimes used, with maltol being a potential option for those who cannot tolerate salt-based oral iron.

Comparison of Common Oral Iron Supplements


Feature	Ferrous Sulfate	Ferrous Bisglycinate	Ferrous Fumarate	Iron Polysaccharide Complex
Bioavailability	Good	High	Good	Moderate
Side Effects	High incidence of GI issues like constipation and nausea	Low incidence of GI issues	Fewer GI side effects for some individuals compared to sulfate	Fewer GI side effects compared to ferrous salts
Cost	Most cost-effective	Higher cost	Moderate	Higher cost
Form	Tablets and liquids	Capsules	Tablets	Capsules, syrup

How to Maximize Iron Absorption

Maximizing iron absorption is crucial.

Combine with Vitamin C: Vitamin C significantly improves the absorption of non-heme iron.
Take on an Empty Stomach: For optimal absorption, take supplements an hour before or two hours after eating. If stomach upset occurs, a small amount of food can help.
Avoid Inhibitors: Certain substances interfere with iron absorption, including dairy products, calcium supplements, antacids, caffeine, and high-fiber foods. Avoid taking them concurrently with your iron supplement.
Consider Alternate-Day Dosing: Taking iron every other day may increase overall absorption and reduce side effects.

Managing Side Effects

To manage common side effects:

Constipation: Increase fiber and water intake, or consider a stool softener.
Nausea and Stomach Pain: Try taking iron with food or switch to a gentler form like bisglycinate.
Dark Stools: This is a normal, harmless side effect.

How Long Should You Take an Iron Supplement?

Treatment duration depends on the deficiency's severity. While symptoms may improve quickly, replenishing iron stores takes longer. Supplementation typically continues for 3 to 6 months after hemoglobin levels normalize to build ferritin stores. Follow-up blood tests will monitor progress.

Conclusion: Choosing the Right Supplement for You

Selecting the best supplement for low iron requires balancing absorption and tolerability. Ferrous sulfate is effective and affordable but can cause more side effects. Gentler options like ferrous bisglycinate offer high absorption with fewer GI issues. Always consult a healthcare professional for diagnosis, personalized dosage, and to choose a supplement you can take consistently to restore iron levels. Resources from organizations like the National Institutes of Health provide further information.(https://ods.od.nih.gov/factsheets/Iron-Consumer/)

Disclaimer: Always consult with a healthcare provider before starting any new supplement regimen.

Frequently Asked Questions

Ferrous bisglycinate (chelated iron) is generally considered the easiest on the stomach. It is bound to amino acids, which makes it less likely to cause constipation and other gastrointestinal side effects compared to traditional ferrous salts like ferrous sulfate.

Taking iron with food can reduce stomach upset and other side effects, but it can also decrease absorption. For best absorption, take it on an empty stomach; if side effects are an issue, take it with a small amount of food or try a gentler formulation.

Vitamin C significantly enhances the absorption of non-heme iron (the type found in supplements). It helps convert ferric iron into the more easily absorbed ferrous iron, maximizing the benefit of your supplement.

You should avoid taking iron supplements with dairy products, calcium supplements, antacids, coffee, and black or green tea. These contain compounds that can inhibit iron absorption, so space them out by at least two hours.

Emerging research suggests that alternate-day dosing, especially for higher doses, may increase the fraction of iron absorbed and reduce gastrointestinal side effects for some individuals. Your doctor can help determine the best schedule for you.

You may start to feel a reduction in fatigue and other symptoms within a few weeks. However, it takes much longer to fully restore your body's iron stores, typically 3 to 6 months. You will need follow-up blood tests to confirm your levels are restored.

Yes, taking excessive amounts of iron can be harmful and toxic, potentially leading to organ damage. It is crucial to only take iron supplements under the guidance of a healthcare provider and to not exceed the recommended dose.