Drugs that suppress the immune system are widely used. They are part of the treatment of patients with organ transplants, malignancy, and increasingly those with conditions such as psoriasis, rheumatoid arthritis, and liver and bowel disease in which inflammation is an aetiological factor. Because of the broadening indications for immunosuppressive drugs, and the prolonged survival in conditions for which they are being used, many patients on immunosuppression are now cared for in the community or seen in non-specialist hospitals, usually in close collaboration with a specialist. This article looks at five commonly used immunosuppressive drugs in turn (corticosteroids, cyclosporin, azathioprine, methotrexate, cyclophosphamide), discussing the main, non-infection, unwanted effects, ways to avoid them and what to do if problems arise. The management of infection is dealt with as a separate section.