In the last two decades or so the intramuscular administration of botulinum toxin type A, and more recently type B, has become an established first line treatment of many neurological and other medical disorders. So far, the toxin has been used mainly by experienced researchers and clinicians with extensive knowledge of its mode of action and potential adverse effects. However, in the foreseeable future it is likely that this treatment will be provided by more medical practitioners and in different clinical settings, especially as the range of its clinical indications increases. Botulinum toxin, in therapeutic doses, is a remarkably safe drug with relatively few adverse effects. The commonest adverse effects are muscle weakness, fatigue, flu-like symptoms, a dry mouth, dizziness and a skin rash. Nonetheless, serious adverse events may occur, albeit rarely, and it is imperative that prescribers of this treatment are thoroughly familiar with its potential risks. The purpose of this article is to review the possible adverse effects of botulinum toxin intramuscular injections, to describe the factors that might predispose to them and to summarise the strategies for their prevention and treatment.