Medication overuse headache (MOH) is a common medical condition that is associated with considerable long-term morbidity and disability. Patients experiencing MOH have primary headache disorders (migraine, tension-type headache [TTH] or the combination of migraine and TTH) that change to a pattern of daily or near-daily headaches over a period of years or decades following the overuse of symptomatic headache medications. Overused drugs include analgesics, ergot alkaloids, serotonin 5-HT(1B/1D) receptor agonists ('triptans') and medications containing barbiturates, codeine, caffeine, tranquillisers and mixed analgesics. Affected patients usually have a long history of primary headache, overuse of medications and MOH before they consult a physician for care. Patients with MOH are usually managed in specialist centres by withdrawal of the overused drugs and treatment of withdrawal symptoms (on an inpatient or outpatient basis), headache prophylaxis and limited use of symptomatic acute medications. Most patients respond to this therapy, although the prognosis is not always good and >or=50% may lapse over an initial 5-year follow-up period. The best practical strategy at present is to prevent the overuse of drugs in the first place by patient education and formal management approaches conducted in primary care to treat the primary headache before it changes to MOH. The quality of the clinical evidence on MOH is suboptimal and further biological and clinical research is urgently required to help facilitate the management of these patients more effectively in the future.