Ferrous Sulfate or Iron Bisglycinate: Which Iron Supplement is Better?

December 13, 2025 •

4 min read

Affecting an estimated 2 billion people worldwide, iron deficiency is a prevalent nutritional disorder. When choosing a supplement, the decision often comes down to the traditional option, ferrous sulfate, or the newer, more bioavailable iron bisglycinate.

Quick Summary

This article compares ferrous sulfate and iron bisglycinate, highlighting that the latter offers superior absorption and better digestive tolerance, while the former is a cost-effective yet often harsher alternative.

Key Points

Bioavailability: Iron bisglycinate offers superior absorption compared to ferrous sulfate due to its chelated structure, requiring a smaller elemental dose for similar results.
Tolerability: Iron bisglycinate is significantly gentler on the stomach, causing fewer side effects like constipation, nausea, and stomach cramps, which are common with ferrous sulfate.
Cost: Ferrous sulfate is generally the more affordable and widely available supplement, while iron bisglycinate represents a more premium option.
Mechanism: The chelated structure of iron bisglycinate protects it from dietary inhibitors, ensuring more consistent absorption, unlike ferrous sulfate which can be affected by foods and drinks.
Compliance: Due to its better tolerability, iron bisglycinate can improve patient compliance, leading to more consistent and successful treatment of iron deficiency.
Expert Opinion: Health experts and studies often recommend iron bisglycinate for patients with sensitive stomachs or poor absorption issues.

Understanding Ferrous Sulfate: The Traditional Approach

Ferrous sulfate is an inorganic iron salt, long-established as a standard treatment for iron deficiency anemia. It is widely available, inexpensive, and effective at raising iron levels. However, its effectiveness is often hampered by significant gastrointestinal side effects, which can reduce patient compliance.

How Ferrous Sulfate Works

When you ingest ferrous sulfate, the body must first break down the salt to release the iron for absorption. This process can cause a high concentration of free iron ions in the digestive tract, which leads to irritation and the common side effects associated with it. Its absorption can also be inhibited by other dietary components like phytates, tannins, and calcium, requiring careful timing of doses.

Side Effects of Ferrous Sulfate

Common side effects include:

Nausea and stomach cramps
Constipation or diarrhea
Heartburn
Dark or black stools
A metallic taste in the mouth

Understanding Iron Bisglycinate: The Modern Alternative

Iron bisglycinate is a newer, chelated form of iron where an iron molecule is bonded to two molecules of the amino acid glycine. This unique structure protects the iron, allowing it to pass through the digestive system more gently and be absorbed more efficiently, with less interference from dietary inhibitors. It is often marketed as a “gentle iron” supplement.

How Iron Bisglycinate Works

The chelated structure allows the iron to be absorbed intact through a different pathway than conventional iron salts, specifically via protein channels for amino acids. This prevents the release of free iron ions in the gut, dramatically reducing gastrointestinal side effects. Studies suggest its bioavailability can be several times higher than ferrous sulfate, meaning a lower dosage may be just as effective.

Side Effects of Iron Bisglycinate

Because it is gentler on the stomach, iron bisglycinate is less likely to cause gastrointestinal distress. While not side-effect-free, it is significantly better tolerated than ferrous sulfate, with adverse reactions being much less frequent and severe. Potential side effects are similar but milder and less common:

Mild nausea or stomach discomfort
Constipation or diarrhea (less common)
Dark stools (a harmless result of iron absorption)

Key Differences: Ferrous Sulfate vs. Iron Bisglycinate


Feature	Ferrous Sulfate	Iron Bisglycinate
Cost	Generally low-cost and widely available.	Typically more expensive due to advanced formulation.
Bioavailability	Lower absorption; inhibited by phytates, tannins, and calcium.	Higher absorption; chelation protects it from dietary inhibitors.
Gastrointestinal Effects	High likelihood of nausea, constipation, cramps, and upset stomach.	Significantly fewer and milder side effects; very gentle on the stomach.
Elemental Iron	Often prescribed in higher doses to compensate for poor absorption.	Effective at lower doses due to superior absorption.
Mechanism	Simple iron salt that can irritate the gut lining upon release.	Chelated compound absorbed intact via amino acid channels.
Drug Interactions	Interactions with many medications and food compounds.	Fewer interactions and less prone to inhibition by food.

Choosing Between Ferrous Sulfate and Iron Bisglycinate

The best choice depends on your individual needs, tolerance, and budget. For those who tolerate standard iron supplements without issue and are cost-sensitive, ferrous sulfate remains a viable and effective option. For most people, however, iron bisglycinate presents a clear advantage.

When to Consider Iron Bisglycinate

Sensitive Stomach: If you have a history of nausea, constipation, or other digestive issues with supplements, iron bisglycinate is the more comfortable and gentle option.
Seeking Optimal Absorption: For faster and more efficient iron replenishment, especially when dietary inhibitors are a concern, its superior bioavailability is a major benefit.
Better Compliance: Fewer side effects lead to better adherence to the treatment plan, which is critical for correcting iron deficiency.

When to Consider Ferrous Sulfate

Budgetary Constraints: As a more economical choice, it is a practical option for those for whom cost is a primary concern.
Proven Tolerance: If you have taken ferrous sulfate previously without any adverse side effects, there is no reason to switch.

Conclusion: Which is the Winner?

While both are effective at treating iron deficiency, iron bisglycinate is generally the superior choice for most individuals due to its significantly higher bioavailability and vastly better digestive tolerance. Its chelated form means less gastrointestinal distress and higher absorption, which can lead to faster and more comfortable iron level restoration. Ferrous sulfate, though a tried-and-true option, is often plagued by uncomfortable side effects that can hinder consistent use. Ultimately, the best option is the one you can take consistently, but for an optimal experience with fewer side effects, iron bisglycinate has a clear advantage.

Disclaimer: Always consult a healthcare professional before starting any new supplement regimen to determine the appropriate treatment for your specific needs.

Frequently Asked Questions

Yes, iron bisglycinate is widely regarded as being significantly gentler on the digestive system. Its chelated form prevents the release of free iron ions that can cause irritation, resulting in fewer instances of nausea, constipation, and cramps.

Studies show that iron bisglycinate has higher bioavailability and is absorbed more efficiently than ferrous sulfate. Its chelated structure protects the iron from dietary inhibitors, leading to better overall absorption.

Ferrous sulfate is a traditional, inorganic iron salt that is less complex to manufacture than chelated iron bisglycinate. Its widespread availability and low production cost make it a budget-friendly option.

Both ferrous sulfate and iron bisglycinate are typically available over-the-counter. However, it is crucial to consult a healthcare provider before starting any iron supplement to confirm a deficiency and determine the correct dosage.

While ferrous sulfate is often recommended with food to minimize stomach upset, this can also inhibit absorption. Iron bisglycinate is less prone to interactions, so it can often be taken with or without food, though taking it with vitamin C can boost absorption.

While you may begin to feel less fatigued within a few weeks, it takes much longer to fully replenish your body's iron stores. Treatment can last for several months, with your doctor monitoring levels via blood tests.

Chelated iron, like iron bisglycinate, is an iron molecule bonded to a ligand, such as an amino acid. This process protects the mineral and improves its absorption by the body, making it gentler on the digestive tract.

Dark or black stools are a common and harmless side effect of iron supplementation, occurring when unabsorbed iron passes through the digestive system. While both forms can cause this, it's generally less pronounced with the better-absorbed iron bisglycinate.