Many patients admitted for surgery are receiving regular drug therapy. Adverse effects may occur, either as a result of these drugs being stopped suddenly or because staff are unaware of significant interactions between certain drugs and anaesthetic agents or techniques. This study aimed to find out how regular drug medication is actually given in the peri-operative period. In addition, pharmaceutical companies were contacted and asked for information about the effects of sudden withdrawal of their products and potential interactions with anaesthetic agents. We found that many drugs were omitted peri-operatively with potentially significant effects. Pharmaceutical companies do not seem to appreciate the importance of this problem and not all of them give clear recommendations relevant to practising anaesthetists.