CYP2D6*23 is assigned uncertain function due to inadequate evidence. In vitro studies found CYP2D6*23 demonstrated modest decreased enzyme activity compared to wildtype with tamoxifen, bufuralol, dextromethorphan, and primaquine substrates (24647041, 30366777). Consensus was not achieved among experts as data were sparse and it was not convincing the magnitude of decreased activity was clinically significant. Therefore, uncertain function was assigned.