This chapter covers patient-specific issues related to children, older people, injecting drug users, and surgical patients. For children, drug pharmacokinetics and pharmacodynamics and dose calculations are reviewed. The specific concerns around adherence and also medicines licensing in children are covered. For older people, the topics include pharmacokinetics, pharmacokinetics, and medication review. Guidance is given on managing injecting drug users, especially in an in-patient setting, including a suggested regimen for titration of methadone to avoid opioid withdrawal. For surgical patients, the topics covered include management of regular medicines during the nil-by-mouth period, drug interactions with perioperative drugs, and a brief description of the impact of selected drugs in the perioperative period.