Adverse events following intravesical BCG therapy are related to strain virulence, allergic reactions or to nosocomial urinary tract infections. Low grade fever and irritative symptoms are common side-effects of BCG. They subside within 48 hours and do not require any specific treatment, apart from standard painkillers and antispasmodics. Further instillations should be postponed until symptoms have resolved completely. If symptoms do not resolve, complementary investigations are recommended including urine culture, and isoniazid may be prescribed for 15 days. The BCG dose should be reduced if symptoms increase after subsequent instillations. Complications of BCG infection – either local or systemic – have been reported with an incidence of 10–15%. These complications include: granulomatous prostatitis or epididymitis (treated with isoniazid and rifampicin for 3 months), contracted bladder may occur, mainly during maintenance courses, systemic infection such as granulomatous nephritis and abscesses, pneumonitis, hepatitis, osteomyelitis (treated with isoniazid, rifampicin and ethambutol for 6 months), and life-threatening adverse events may be related to septicaemia or to immunoallergic reactions, the onset of which may be delayed several months after the end of BCG therapy. Such conditions require urgent treatment with standard antituberculous antibiotics and prednisolone. These complications are an absolute contraindication for further BCG instillations. Despite its toxicity, the risk-benefit ratio favours the use of BCG in patients who have moderate- and high-risk tumours.