Patients with hematological malignancies receiving hematopoietic stem cell transplantation (HSCT) or chimeric antigen receptor (CAR) T-cell therapy are known to have complex pharmacotherapy. To ensure safe and effective care in preventing and managing drug-related problems, pharmacists trained in HSCT and CAR T-cell therapy fulfill various roles in providing care for these patients. With polypharmacy and complex medication regimens, drug interactions are one of the major aspects that pharmacists review regularly to ensure the efficacy of treatment while minimizing toxicity. Pharmacists anticipate various complex drug interactions, understanding the possible mechanisms of drug interactions, and manage these complex drug regimens in patients undergoing HSCT and CAR T-cell therapy. In addition, antimicrobial prophylaxis is also important supportive care for immunocompromised patients. Pharmacists are capable of evaluating the risk of opportunistic infections, anticipating the type of antimicrobial prophylaxis required, and setting a policy with a robust antimicrobial regime. This can promote consistency in patient care and improve patient outcomes in terms of morbidity and mortality associated with opportunistic infections. Lastly, pharmacists are equipped with a skillset to ensure a seamless transition of care, provide education for patients and healthcare providers, promote medication adherence, and contribute to research and quality improvement.