Genetic Variability and Personalized Drug Response: Toward Implementation of Pharmacogenomics in Clinical Practice

Interindividual variability in drug response remains a major challenge in clinical pharmacotherapy, contributing to suboptimal efficacy or serious adverse drug reactions. Pharmacogenomics — the study of how genetic variation affects drug response — offers the potential to tailor treatment based on a patient’s unique genetic profile. This article reviews evidence that genetic polymorphisms in drug‑metabolizing enzymes (e.g., cytochrome P450 family), transporters, and drug targets underlie substantial differences in pharmacokinetics and pharmacodynamics across populations. We discuss well-characterized pharmacogenes such as CYP2D6, CYP2C19, and CYP3A4 — their variant alleles and how these influence drug clearance, activation of prodrugs, or risk of toxicity. Advances from candidate-gene studies to genome‑wide and rare‑variant analyses are described. Challenges hindering clinical implementation — including limited evidence for many variants, rare variants’ contribution, polygenic effects, and lack of consensus guidelines — are addressed. We propose methodological strategies for integrating pharmacogenomic testing into clinical workflows and highlight potential benefits for personalized medicine, including improved efficacy, reduced adverse events, and cost‑effective therapy. Finally, we outline future directions, such as incorporating rare variants, using high‑throughput sequencing, and developing robust clinical decision support.