Humans have considerable genetic variation, as shown by their DNA sequence differences. Drug responsiveness and toxicity are affected by this variability. Genetic variation affects prodrug activation, drug targets, downstream activation pathways, drug elimination, and toxicity activation. Molecular diagnostic methods have discovered the genetic basis of several of these "outlier" drug responses. By predicting a patient's response, such diagnostics can improve both the safety and efficacy of drugs. The article illustrates the matching of molecular diagnosis to drug therapy for improved patient outcomes.