Drug interactions occur between a pharmaceutical agent and a substance such as another drug. They are clinically relevant because adverse drug reactions (ADRs) lead to patient harm. This is particularly important in anaesthetic and intensive care practice due to the multiplicity of pharmaceuticals employed. As the patient demographic continues to evolve with an increasingly vulnerable population, e.g. elderly patients with comorbidities and organ transplant recipients, clinicians are more likely to encounter polypharmacy and ADRs. Knowledge and understanding of agents taken regularly or recreationally and anticipation of potential interactions with subsequent prescribing can avoid harmful interactions.