The cytochrome P450 (CYP) enzyme system influences metabolism of psychotropic medications including antidepressants and antipsychotics. Several companies now market tests that utilize multiple genetic markers reflecting the status of these metabolic enzymes. Academic laboratories and genetic testing companies have designed proprietary algorithms to predict the combined effect of these variants on drug response and side effects.50 The efficacy of these pharmacogenomic profiles requires further investigation in controlled studies. However, several hundred thousand tests have already been performed commercially, and the costs may be covered by third-party payers. Two specific enzymes have figured in FDA warnings. Patients who are slow metabolizers based on CYP2C19 should not receive doses of citalopram greater than 20 mg daily.51 There is a similar warning for doses of vortioxetine greater than 10 mg for slow metabolizers based on CYP2D6.52
