Can iron bisglycinate cause black stools?

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Last updated: October 8, 2026 • View editorial policy

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Iron Bisglycinate and Black Stools

Iron bisglycinate can cause black stools, but this occurs significantly less frequently than with traditional ferrous iron salts like ferrous sulfate or ferrous fumarate.

Frequency of Black Stools with Iron Bisglycinate

The most direct evidence comes from a randomized study comparing equipotent doses of different iron formulations in pregnant women 1. Ferrous bisglycinate 25 mg caused black stools in only 8% of patients, compared to 22% with ferrous fumarate 40 mg and 31% with ferrous sulphate 50 mg 1. This represents a roughly 3-4 fold lower incidence compared to traditional iron salts.

Why the Difference Matters Clinically

The chelated structure of iron bisglycinate results in:

  • Better absorption - more iron is absorbed rather than remaining in the gut lumen
  • Less unabsorbed iron in stool - which is what causes the characteristic black discoloration
  • Improved GI tolerability overall - the same study showed bisglycinate had significantly fewer total GI complaints 1

Practical Counseling Points

When prescribing iron bisglycinate, inform patients that:

  • Black stools can still occur but are much less common than with ferrous sulfate
  • The absence of black stools does not mean the supplement isn't working
  • If black stools do appear, this is a harmless side effect from unabsorbed iron
  • The frequency of black stools increases with higher iron doses across all formulations 1

Clinical Context from Guidelines

While the major gastroenterology guidelines 2 acknowledge ferrous bisglycinate as an available oral iron option, they note there is no substantial evidence that any one oral iron product is superior in effectiveness or tolerance in clinical trials 2. However, the guideline does state "some patients may tolerate other products such as ferrous bisglycinate better" 2.

The 2024 AGA guideline 2 lists ferrous bisglycinate as containing approximately 25 mg elemental iron per tablet, making it a lower-dose option that aligns with emerging evidence suggesting lower or alternate-day dosing may improve tolerability without sacrificing efficacy.

Bottom Line

Yes, iron bisglycinate can turn stools black, but expect this in less than 10% of patients - a marked improvement over the 20-30% rate seen with traditional ferrous salts at equipotent doses. This makes it a reasonable alternative for patients who experience intolerable GI side effects with standard iron formulations, though it may cost more 2.

References

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.