CYP2D6*18 is assigned no function based on moderate evidence. CYP2D6*18 is defined by a 9 base insertion yielding a nonfunctional enzyme and has been described in subjects demonstrating markedly decreased metabolism of sparteine and debrisoquine (8946471, 10471060). Multiple in vitro studies found CYP2D6*18 had decreased enzyme activity compared to wildtype with various substrates (8946471, 18784265, 24647041, 30366777). Therefore, consensus among experts was no function with an activity value of 0 based on moderate evidence.