Randomised controlled trials that compared anaesthesia with and without nitrous oxide in adults (18 years or over), and reported data on postoperative nausea and vomiting, were eligible for inclusion. Trials in which nitrous oxide was used as a sedative or analgesic agent were excluded, as were trial with no cases of nausea and vomiting in any group.
Included trials compared the non-nitrous oxide anaesthetics alfentanil, desflurane, enflurane, fentanyl, isoflurane, ketamine, morphine, propofol, sevoflurane and sufentanil (alone or in varying combinations) with the same anaesthetic combined with nitrous oxide. Types of surgery included oral, gynaecological, laparoscopy, general, neurological, vascular, orthopaedic, and ear-nose-throat.
Two reviewers independently assessed studies for inclusion; disagreements were resolved through discussion.