A 62-year-old man with schizophrenia has shown inadequate response to multiple antipsychotics at standard doses despite confirmed medication adherence via plasma level monitoring, which shows consistently low CNS-to-plasma ratios. Pharmacogenomic testing reveals he is homozygous for a gain-of-function variant of the ABCB1 gene encoding P-glycoprotein. The PMHNP considers how this finding affects drug delivery to the brain.