Long-term treatment with vitamin K antagonists (vKA) frequently involves complications. The commonest complication is haemorrhage and cases of serious haemorrhage are stated in the literature to occur with a frequency per 1,000 treatment years of 12-108, of which 2-17 are fatal. The majority of deaths associated with haemorrhage are due to intracranial haemorrhage. Notifications of complications of vKA treatment in Denmark are considerably fewer than might be anticipated from the literature. The stability of anticoagulation treatment decreases with the number of drugs administered simultaneously and numerous drugs and other factors interact with the effect of vKA. A series of examples are reviewed and some known drugs which do not interact are mentioned. Non-haemorrhagic side effects of coumarin derivatives are rare. Anticoagulation treatment during pregnancy is associated with very special problems and must be regarded as a specialist task.