A 55-year-old woman with major depressive disorder is started on nortriptyline 75 mg at bedtime. After 10 days she develops severe sedation, orthostatic hypotension, dry mouth, and a QTc interval of 510 ms. A pharmacogenomic panel reveals she is a CYP2D6 poor metabolizer. The PMHNP integrates these results into the treatment plan. Which of the following best explains the clinical significance of this patient's pharmacogenomic result?