Can other drugs interfere with hormone therapy?

Certain drugs, including several commonly prescribed antidepressants (those in the category called selective serotonin reuptake inhibitors, or SSRIs), inhibit an enzyme called CYP2D6. This enzyme plays a critical role in the use of tamoxifen by the body because it metabolizes, or breaks down, tamoxifen into molecules, or metabolites, that are much more active than tamoxifen itself.

The possibility that SSRIs might, by inhibiting CYP2D6, slow the metabolism of tamoxifen and reduce its potency is a concern given that as many as one-fourth of breast cancer patients experience clinical depression and may be treated with SSRIs. In addition, SSRIs are sometimes used to treat hot flashes caused by hormone therapy.

Researchers have found that women taking certain SSRIs together with tamoxifen have decreased blood levels of active tamoxifen metabolites. Because of this, many experts suggest that patients who are taking antidepressants along with tamoxifen should discuss treatment options with their doctors.


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For example, doctors may recommend switching from an SSRI that is a potent inhibitor of CYP2D6 (such as paroxetine) to one that is a weaker inhibitor (such as sertraline) or that has no inhibitory activity (such as venlafaxine or citalopram), or they may suggest that their postmenopausal patients take an aromatase inhibitor instead of tamoxifen.

Other medications that inhibit CYP2D6 include the following:

  • Quinidine, which is used to treat abnormal heart rhythms
  • Diphenhydramine, which is an antihistamine
  • Cimetidine, which is used to reduce stomach acid

People who are prescribed tamoxifen should discuss the use of all other medications with their doctors.

Where can someone find more information about drugs used in hormone therapy for breast cancer?

NCI’s Drug Information Summaries provide consumer-friendly information about certain drugs that are approved by the FDA to treat cancer or conditions related to cancer. For each drug, topics covered include background information, research results, possible side effects, FDA approval information, and ongoing clinical trials. The Drug Information Summaries include information about drugs that have been approved for breast cancer.


Selected References

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Source: National Cancer Institute