CYP2D6 and serotonin syndrome: Furthermore, studies have revealed that serotonin syndrome occurrence may be associated with the polymorphisms of the T102C site of the 5-HT2A receptor and CYP2D6.[28–31] Moreover, serotonin syndrome occurrence is closely associated with the patient medication, disease, and genetic status, with a low incidence; however, when combined with medication for patient cancer pain, its incidence considerably increases.