Drugs Causing Black/Dark Urine Discoloration

1. Iron Sorbitol (IM injection)

• The classic exam answer for jet-black urine
• Given intramuscularly, it is excreted via the kidneys and turns urine black
• Oral iron preparations do NOT cause this effect (they cause black stools instead)

2. Levodopa / L-DOPA

• Oxidation of levodopa metabolites in urine produces dark brown to black coloration
• This is a benign side effect with no clinical harm

3. Methyldopa (Alpha-methyldopa)

• Similar catecholamine oxidation mechanism to levodopa
• Causes darkening of urine on standing/exposure to air

4. Metronidazole

• A well-documented but underreported side effect
• Mechanism: an azometabolite pigment is responsible for the dark/black urine
• PMC case report (2014) confirmed this is a probable adverse drug reaction - urine normalizes on stopping the drug and darkens again on restarting it

5. COMT Inhibitors (Entacapone, Tolcapone)

• Used in Parkinson's disease alongside levodopa
• Cause orange-brown to dark urine discoloration due to accumulation of a metabolite
• Specifically noted in Harrison's (2025): "Discoloration of urine can be seen with COMT inhibitors due to accumulation of a metabolite, but it is of no clinical concern" - Harrison's Principles of Internal Medicine 22E

6. Nitrofurantoin

• Causes brownish-black discoloration of urine

7. Cascara / Senna (anthraquinone laxatives)

• Produce reddish-brown to black urine

8. Phenol / Cresol

• Toxic exposure causes black urine

Quick Memory Aid

For MCQ purposes: If the question asks specifically for black urine, the single best answer is Iron Sorbitol (IM). If it asks for drugs "causing dark/black urine" broadly, levodopa, methyldopa, and metronidazole are also correct.