Most common adverse drug reactions (ADRs) are linked to pharmacology. No medicine has a single physiological effect, and many cause unwanted side effects in addition to their intended action, even when administered at the recommended dose. The higher the number of medicines people take, the higher their risk of ADRs. This article explores the role of pharmacokinetics and pharmacodynamics in ADRs and discusses how understanding the way in which the body processes medicines, and the factors that affect these mechanisms, can assist nurses in anticipating and avoiding ADRs. The article also explains how understanding the mode of action before administering medicines to patients can predict and prevent harmful interactions, exaggerated or unwanted side effects and responses at unintended sites.