CYP2C19*34 is assigned uncertain function due to inadequate evidence. CYP2C19*34 was identified in a patient heterozygous for the CYP2C19*2 allele who experienced adverse effects with venlafaxine and a supratherapeutic nortriptyline level. CYP2C19 enzyme function was not adequately assessed as CYP2D6 is involved in metabolism of antidepressants such as nortriptyline and this patient was a CYP2D6 intermediate metabolizer (23799451). Therefore, consensus among experts was uncertain function due to inadequate evidence.