The calculation of odds ratio indicated that a genetic background positive for the presence of genotype CYP2D6 Ht *1/*4 increased three-fold and two-fold the risk for developing MCS and either SMCS or FM/CFS, respectively, while the same risk was 52-fold, 38-fold, and 36-fold increased by the presence of CYP2D6 Ht *1/*41. Here, CYP2D6 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.