Can Anastrozole Cause Dark Urine?
No, anastrozole does not cause dark urine as a recognized adverse effect. Dark urine is not listed among the documented side effects of anastrozole in clinical guidelines or major clinical trials.
Known Adverse Effects of Anastrozole
The well-established adverse effects of anastrozole in postmenopausal women with breast cancer include 1:
- Musculoskeletal symptoms: Joint stiffness, arthralgias, and myalgias are very common (occurring in 30-53% of patients) 1
- Bone-related effects: Increased fracture risk (14% vs 9% with placebo) and new osteoporosis (11% vs 6% with placebo) 1
- Vasomotor symptoms: Hot flashes occur in approximately 38% of patients 1
- Cardiovascular events: Occur in approximately 12% of patients 1
- Gastrointestinal disturbances: Including nausea and diarrhea, though diarrhea is infrequent (4-6%) 1
- Fatigue: Reported in approximately 36% of patients 1
Important Distinction: Drugs That DO Cause Dark Urine
Dark or discolored urine is a well-documented effect of certain other medications, but not anastrozole:
- Rifampin and rifabutin cause universal orange discoloration of bodily fluids including urine, and patients must be warned of this effect at treatment initiation 1
- Metronidazole can rarely cause reddish discoloration of urine 2
- Methimazole can cause dark urine as part of cholestatic jaundice, a rare but serious adverse effect 3
Clinical Approach If Dark Urine Occurs
If a patient on anastrozole develops dark urine, investigate alternative causes 3:
- Hepatobiliary dysfunction: Check liver function tests, bilirubin levels, and assess for jaundice or pruritus
- Hematuria: Perform urinalysis to evaluate for blood in urine
- Dehydration: Assess hydration status and urine concentration
- Concurrent medications: Review all medications for known causes of urine discoloration
- Underlying conditions: Consider urinary tract infection, liver disease, or hemolysis
The development of dark urine in a patient taking anastrozole should prompt evaluation for other causes rather than attribution to the aromatase inhibitor itself.