Adverse drug reactions are unwanted effects from medicines, and these can range from mild or irritating to severe or life-threatening. They are a significant cause of mortality, morbidity and hospital admissions and a common reason why patients do not take their medicines as instructed. Identification of adverse drug reactions can be complex because symptoms are often non-specific and because reactions do not always occur immediately after a medicine is given. A drug interaction is a clinical response that occurs when the effect of a drug is altered by the presence of another drug or other substance. Drug interactions may be classed as ‘pharmacokinetic’ (altering drug disposition) or ‘pharmacodynamic’ (altering drug effect).