MAJOR — Use With Caution
FDA tamoxifen label warns about strong CYP2D6 inhibitors. Reduced endoxifen levels may decrease tamoxifen anticancer efficacy. Use alternative antidepressants.
Evidence level: STRONG
Fluoxetine (Prozac) can make tamoxifen (a breast cancer drug) less effective. Tamoxifen needs to be activated by the CYP2D6 enzyme in your liver, and Prozac blocks this enzyme. This could reduce your protection against breast cancer recurrence.
Fluoxetine is a potent CYP2D6 inhibitor. Tamoxifen is a prodrug requiring CYP2D6 activation to its active metabolite endoxifen. Fluoxetine blocks this activation, potentially reducing tamoxifen anticancer efficacy. FDA tamoxifen label warns about CYP2D6 inhibitors.
If you take tamoxifen for breast cancer, avoid Prozac. Several safer antidepressant alternatives exist that do not block CYP2D6 (such as venlafaxine, citalopram, or escitalopram at low doses).
Take Fluoxetine at the same time each day — morning is most common to avoid sleep disruption. Tamoxifen follows its prescribed schedule. SSRIs have a long half-life, so consistency matters more than the exact hour.
Higher risk for: elderly, those on multiple serotonergic drugs, people with liver impairment, CYP2D6 poor metabolizers, recent dose changes, or concurrent use of MAOIs or triptans.
No immediate symptoms — the risk is reduced tamoxifen effectiveness, meaning potentially higher risk of breast cancer recurrence over time
Ask your oncologist about CYP2D6-safe antidepressants. Venlafaxine and citalopram are commonly recommended. Do not switch antidepressants without medical guidance.
Discuss this combination with your prescriber. They may adjust timing, dosing, or switch to an alternative with fewer interaction risks. Never stop an antidepressant abruptly — taper under medical guidance.
Fluoxetine (Prozac) can make tamoxifen (a breast cancer drug) less effective. Tamoxifen needs to be activated by the CYP2D6 enzyme in your liver, and Prozac blocks this enzyme. This could reduce your protection against breast cancer recurrence.
Take Fluoxetine at the same time each day — morning is most common to avoid sleep disruption. Tamoxifen follows its prescribed schedule. SSRIs have a long half-life, so consistency matters more than the exact hour.
No immediate symptoms — the risk is reduced tamoxifen effectiveness, meaning potentially higher risk of breast cancer recurrence over time
Discuss this combination with your prescriber. They may adjust timing, dosing, or switch to an alternative with fewer interaction risks. Never stop an antidepressant abruptly — taper under medical guidance.
Ask your oncologist about CYP2D6-safe antidepressants. Venlafaxine and citalopram are commonly recommended. Do not switch antidepressants without medical guidance.
Or browse the full interaction database (121,000+ pairs).