The relationship between CYP2D6 and hyperprolactinemia (a possible adverse effect of RIS) appears to be U-shaped, with a tendency (though not consistently replicated) for both extremes of CYP2D6 metabolic phenotype (i.e., PMs and UMs) to show an association with hyperprolactinemia (Troost et al., 2007; Roke et al., 2013; Youngster et al., 2014; Sukasem et al., 2016). This evidence concerns the gene CYP2D6 and hyperprolactinemia.