Cancer is caused due to genetic mutations that cause cells to proliferate uncontrollably. Multiple therapies are available to treat cancer, while chemotherapy is mostly effective. It aims to target the rapidly multiplying cells, although patient responses may differ due to genetic differences that influence drug metabolism and transport. Pharmacogenomics is the science that studies the influence of genes on an individual’s response to drugs. Further, it seeks to optimize treatment effectiveness and reduce adverse side effects by modifying therapies based on genetic profiles. Recent research has underscored the importance of pharmacogenomics in oncology. A large-scale analysis detected clinically relevant pharmacogenetic variants in 62.7% of 76,805 cancer patients, highlighting the prevalence of genetic factors that could influence drug response. Additionally, studies have identified specific gene variants, such as those in the DPYD gene, that increase the risk of severe toxicity from certain chemotherapeutic agents, emphasizing the need for genetic screening prior to treatment. The integration of pharmacogenomics testing into clinical practice has already led to significant improvements in patient care. For instance, genetic testing for DPYD variants can guide dosage adjustments for fluoropyrimidine-based chemotherapies, reducing the incidence of adverse reactions. As our understanding of the genetic underpinnings of drug response continues to grow, pharmacogenomics holds the promise of more personalized and effective cancer treatments, ultimately improving patient outcomes. Some pharmacogenetic clinical applications (5-FU, irinotecan, thiopurines) have already been developed in normal medicine, leading to significant improvements in patient care.