Bisphosphonates are widely used to treat osteoporosis, bone complications of cancer, malignant hypercalcaemia and Paget's disease. Osteonecrosis of the jaw is a major adverse effect of these drugs. It can affect eating, interpersonal relations, and physical appearance. Ulceration of the tongue and nerve involvement can also occur. Osteonecrosis of the jaw due to bisphosphonates is mainly seen in cancer patients, and more rarely in patients treated for osteoporosis. Between 5% and 11% of cancer patients treated with bisphosphonates develop osteonecrosis of the jaw. The frequency of jaw osteonecrosis increases with the duration of bisphosphonate treatment and is higher in case of recent dental work. The main bisphosphonate withdrawal and surgery are rarely effective. In practice, when bisphosphonate therapy is needed, the patient must be informed of the risks, should seek dental care (if possible) before starting treatment, see a dentist regularly, and consult a doctor in case of delayed healing after dental extraction, jaw pain, or numbness.