Does Cotrimoxazole Cause Red Urine?
No, cotrimoxazole (trimethoprim-sulfamethoxazole) does not cause red urine as a recognized adverse effect.
Evidence from FDA Drug Labeling
The FDA-approved prescribing information for trimethoprim-sulfamethoxazole does not list red or discolored urine as an adverse effect 1. The drug is primarily excreted renally, with 84.5% of total sulfonamide and 66.8% of free trimethoprim recovered in urine, but this excretion does not result in red discoloration 1.
Known Adverse Effects Related to Urine
The documented genitourinary adverse effects of cotrimoxazole include 2:
- Renal failure
- Interstitial nephritis
- Elevated blood urea nitrogen and serum creatinine
- Toxic nephrosis with oliguria and anuria
- Crystalluria (crystal formation in urine)
- Nephrotoxicity when combined with cyclosporine
Recent evidence confirms that sulfamethoxazole can form N-acetyl-sulfamethoxazole (NASM) crystals in acidic urine, potentially causing crystalline nephropathy and acute kidney injury 3. However, these crystals do not produce red urine.
Important Clinical Considerations
If a patient on cotrimoxazole presents with red urine, consider alternative explanations:
- Hematuria from crystalline nephropathy or interstitial nephritis (actual blood in urine, not discoloration from the drug itself) 3
- Hemolytic anemia with hemoglobinuria - cotrimoxazole can rarely cause severe hemolytic anemia 4, which could result in dark or red-tinged urine from hemoglobin breakdown products
- Concurrent medications or conditions causing urine discoloration
- Urinary tract infection or other pathology
Clinical Action
If red urine develops during cotrimoxazole therapy:
- Perform urinalysis to distinguish between hematuria and discoloration
- Check complete blood count to evaluate for hemolytic anemia 4
- Assess renal function (creatinine, BUN) 3
- Evaluate for crystalluria, particularly if urine pH is acidic 3
- Consider drug discontinuation if hemolytic anemia or significant nephrotoxicity is present
The red urine is a sign of potential serious adverse effects (hematuria from nephrotoxicity or hemoglobinuria from hemolysis), not a benign drug-related discoloration.