I am taking ferrous sulfate for iron‑deficiency anemia and notice black or tarry stools; is this a normal side effect or a sign of gastrointestinal bleeding?

Black Stool from Ferrous Sulfate: Normal Side Effect vs. Gastrointestinal Bleeding

Black or tarry stools during ferrous sulfate therapy are an expected, benign side effect caused by unabsorbed iron passing through the colon—not a sign of gastrointestinal bleeding. 1

Understanding Iron-Induced Stool Changes

• Only a small percentage of oral iron is absorbed in the small intestine; the majority passes unabsorbed into the colon, where it oxidizes and turns stool dark or black 1
• The frequency of black stools increases with iron dose: ferrous bisglycinate 25 mg causes black stools in 8% of patients, ferrous fumarate 40 mg in 22%, and ferrous sulfate 50 mg in 31% 2
• This color change is a direct result of unabsorbed elemental iron reaching the colon and does not indicate bleeding 1

Distinguishing Benign Iron Stool from Gastrointestinal Bleeding

Key clinical differences:

• Iron-induced black stool is uniformly dark throughout, has a consistent color, and occurs predictably after starting iron supplementation 1
• Melena from GI bleeding is typically tarry, sticky, foul-smelling, and associated with other symptoms such as weakness, dizziness, abdominal pain, or hemodynamic instability 3
• If you stop taking iron for 48–72 hours and the black color resolves, this confirms the stool change was due to iron supplementation rather than bleeding 1

When to Investigate for Gastrointestinal Bleeding

Pursue urgent evaluation if:

• Black stools are accompanied by new-onset weakness, lightheadedness, syncope, or tachycardia 3
• Hemoglobin fails to rise by at least 10 g/L after 2 weeks of daily oral iron therapy, which strongly predicts treatment failure and suggests ongoing blood loss (sensitivity 90.1%, specificity 79.3%) 3, 4
• Abdominal pain, vomiting, or other gastrointestinal symptoms develop alongside black stools 3
• Hemoglobin continues to decline despite adherent iron therapy 3

Minimizing Stool Discoloration While Maintaining Efficacy

• Lower-dose iron formulations produce less stool discoloration: ferrous bisglycinate 25 mg daily causes black stools in only 8% of patients compared to 31% with ferrous sulfate 50 mg 2
• Alternate-day dosing with 100–200 mg elemental iron significantly increases fractional absorption while reducing gastrointestinal side effects, including stool changes 3, 4
• Once-daily dosing of 50–100 mg elemental iron is superior to multiple daily doses and minimizes unabsorbed iron reaching the colon 3, 4

Common Pitfalls to Avoid

• Do not discontinue iron therapy solely because of black stools, as this is an expected and harmless effect 1
• Do not assume all black stools are benign in patients with risk factors for GI bleeding (e.g., NSAID use, anticoagulation, history of ulcers) 3
• Do not ignore failure of hemoglobin to rise within 2–4 weeks, as this strongly suggests ongoing blood loss or malabsorption requiring investigation 3, 4
• Do not confuse iron-induced gastritis (a rare complication of chronic high-dose iron therapy) with simple stool discoloration; iron gastritis presents with epigastric pain, nausea, and endoscopic findings 5

Monitoring and Follow-Up

• Check hemoglobin at 4 weeks to confirm adequate response (expected rise of approximately 1 g/dL within 2 weeks) 4, 1
• If hemoglobin does not increase by at least 10 g/L after 2 weeks, investigate for non-compliance, continued bleeding, malabsorption, or concurrent vitamin B12/folate deficiency 3, 4
• Continue iron therapy for approximately 3 months after hemoglobin normalizes to replenish iron stores 4, 1