One challenge with clinically applying pharmacogenomic information relates to a lack of guidance of how one should use this information, even though multiple approaches have been taken to establish guidelines. In 2001, Kirchheiner et al. published a comprehensive review of available pharmacogenomic studies of antidepressant drugs metabolized by CYP2D6 and CYP2C19 enzymes.18 Based on these data (representing data published between 1997 and 2003), the authors calculated a percentage dose adjustment recommended for each metabolizer category. This publication was one of the first that aggregated information to highlight and provide pharmacokinetic justification for dosage adjustments. This information was provided in a way that suggested drug-specific dosage adjustments based on changes in pharmacokinetic parameters known to result from differences in metabolizer status (e.g., PMs of CYP2C19 should receive 60% of the average dose of citalopram). In 2006, de Leon et al. published clinical guidelines for psychiatrists regarding pharmacogenomic testing for CYP2D6 and CYP2C19.19 The authors provided general information about the availability of genetic testing, information about choice of appropriate test, and dose recommendation for antidepressants and antipsychotic drugs metabolized by CYP2D6 and
