Drug intoxication, acute illness and other stressors can produce delirium, a common complication of hospitalisation in older patients, particularly those with dementia. Because delirium is associated with a high mortality and morbidity, clinicians must recognise it and treat its underlying causes without delay.Drugs are a leading contributor to delirium. Agents commonly linked to delirium include anticholinergic drugs, hypnosedatives, analgesics (opioid and nonopioid), histamine H2receptor antagonists and antiparkinsonian drugs. Drugdrug and drug-disease interactions may also cause delirium in situations when a single drug alone would be well tolerated. For many drugs, it is not known through what mechanism they produce acute mental deterioration. Furthermore, delirium can occur despite ‘therapeutic’ serum concentrations.A major challenge is excluding other medical problems. A cost-effective approach focuses on a basic clinical evaluation searching for the most common aetiologies, such as fluid/electrolyte disturbances, infection and drug toxicity. Specialised testing such as neuroimaging is reserved for selected cases.The key to managing delirium is to treat its underlying cause. If a medication is at fault, eliminating that agent, or substituting a less deliriogenic alternative, is needed. Delirium takes time to abate, so the patient must be kept from harm in the meantime. This includes restorative and supportive care, and control of behaviours that are harmful to the patient or others around them.Pharmacological therapy is often used to manage delirium, but no medication used to treat delirium is entirely safe.