In this case scenario below, I discuss the mirabegron tamoxifen interaction and how it can impact patient safety.
KS is a 61-year-old female who presents with worsening urinary frequency. She has been diagnosed with overactive bladder and place on oxybutynin. She was taking 5 mg daily and states that the dry mouth from the medication is not worth the benefit she is receiving.
Because of this lack of efficacy and adverse effect, she stops taking it and would like another medication. Her PCP discusses the use of mirabegron to help with her symptoms of overactive bladder.
There’s a problem with that prescription, however. The mirabegron tamoxifen interaction can put her at risk for reduced effectiveness of the tamoxifen. Tamoxifen (a prodrug) is metabolized by CYP2D6 into its active form. Mirabegron inhibits CYP2D6, thus preventing conversion to the active metabolite. The net result from this is likely going to be reduced effectiveness of the tamoxifen. Here’s another case from an antidepressant that can inhibit CYPD2D6.
KS has a strong family history of breast cancer and is at significant risk for breast cancer and that is why she is on the tamoxifen. By using mirabegron, we would likely put her at higher risk for breast cancer.
This combination should be avoided in this patient and alternative therapy should be tried. She did not tolerate the oxybutynin, but there are several other bladder anticholinergics that may be worth considering. Looking for more pearls on mirabegron? Listen to one of our recent podcast episodes!
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