Can Letrozole Cause Diarrhea?
Yes, letrozole can cause diarrhea, though it occurs in only about 6% of patients and is typically mild. According to the FDA drug label, diarrhea is a recognized adverse effect of letrozole treatment 1.
Evidence from Clinical Trials
The FDA-approved prescribing information for letrozole clearly documents diarrhea as an adverse reaction:
- In controlled trials comparing letrozole to other treatments, diarrhea occurred in 6% of patients taking letrozole 2.5 mg (the standard dose) compared to 5% on megestrol acetate and 4% on aminoglutethimide 1
- The diarrhea associated with letrozole was generally mild to moderate in severity
- Most adverse reactions, including diarrhea, could not always be definitively distinguished from consequences of the underlying breast cancer, effects of estrogen deprivation, or other concurrent illnesses 1
Research studies confirm this finding. In a Phase III trial comparing letrozole to megestrol acetate in advanced breast cancer, letrozole groups were more likely to experience diarrhea compared to the comparator group 2.
Clinical Context
While diarrhea is a documented side effect of letrozole:
- The incidence is relatively low (6%) compared to other aromatase inhibitor side effects
- It is much less common than the typical side effects you should expect, which include hot flashes, arthralgia/myalgia (joint and muscle pain), and bone loss 3
- The severity is usually manageable without treatment discontinuation
What to Monitor
If you're experiencing diarrhea on letrozole:
- Document the frequency and severity - is it interfering with daily activities?
- Consider other causes - new medications, dietary changes, infections, or disease-related factors
- Report persistent or severe diarrhea to your oncologist, as dose modifications may be needed
- Be aware that the more common side effects to expect are musculoskeletal symptoms (arthralgia in 53-69% of patients) and hot flashes (38-54% of patients) 3
The diarrhea you're experiencing could be related to letrozole, but given its relatively low incidence, other contributing factors should also be considered.