A drug interaction (DI) occurs when the effect of one medication — its efficacy, safety, or both is altered by the presence of another substance taken at the same time. That substance may be a second drug, a component of food, an herbal or dietary supplement, or, in a broader sense, a disease state that changes how a drug behaves in the body. Interactions are not inherently harmful; some are exploited deliberately for therapeutic benefit, such as combining a beta-lactam antibiotic with a beta-lactamase inhibitor. The great majority of clinically important interactions, however, are unintended and carry the potential to reduce therapeutic effect, provoke toxicity, or do both simultaneously.

The clinical relevance of this topic has grown alongside the increasing complexity of modern prescribing. A repeated cross-sectional analysis of community prescribing in Scotland found that, between 1995 and 2010, the proportion of adults dispensed five or more regular drug classes doubled to 20.8%, while the proportion dispensed ten or more nearly tripled to 5.8% and the prevalence of potentially serious drug–drug interactions rose in parallel with this polypharmacy trend (Guthrie et al.). The consequences are not merely theoretical: a recent descriptive analysis of the United States FDA Adverse Event Reporting System identified over 167,000 reported cases of drug–drug interactions, of which more than 44% resulted in hospitalisation and 8.4% resulted in death, with widely prescribed agents such as warfarin, aspirin, sertraline, tacrolimus, and simvastatin among the most frequently implicated medicines (Alahmari et al.). In recognition of this burden, the World Health Organization launched its Third Global Patient Safety Challenge, Medication Without Harm, in 2017, targeting a 50% reduction in severe, avoidable medication-related harm worldwide, with polypharmacy explicitly named as one of three priority action areas (World Health Organization). Because a substantial proportion of interaction-related events are considered preventable, understanding why and how drugs interact is a prerequisite for anticipating and avoiding them, rather than simply reacting to them after harm has occurred.

This review focuses on the general mechanisms by which drug interactions arise and the systems used to classify them. It is organised around three complementary classification lenses mechanism, source, and clinical significance before examining pharmacokinetic and pharmacodynamic mechanisms in detail, the growing role of pharmacogenomics, the patient- and drug-related factors that modulate clinical risk, illustrative high-risk drug classes, methods for detecting and assessing interactions, and general principles of prevention and management.