Pethidine is the most widely used intra-muscular opioid for the relief of labour pain. However concerns have been raised about its effectiveness and the possibility of depressing respiration in newborns.The objective of this review was to assess the effects of different opioids (and different doses of the same opioid) administered intra-muscularly in labour.We searched the Cochrane Pregnancy and Childbirth Group trials register, the Cochrane Controlled Trials Register (Cochrane Library, issue 4, 1997) and reference lists of articles.Randomised trials comparing the effects of different currently used opioids (and different doses of the same opioid) administered intramuscularly in labour for women who request systemic analgesia.Two reviewers assessed trial quality and extracted data. Analysis was based on the groups as randomly allocated.Sixteen trials were included. There were problems with methodological quality of some of the trials, and lack of consistency in the way various outcomes were reported. There was no evidence of a difference between pethidine and tramadol in terms of pain relief, interval to delivery, or instrumental or operative delivery. There appeared to be more adverse effects such as nausea and vomiting and drowsiness with pethidine. Maternal pain relief seemed almost identical between the meptazinol and pethidine groups, whether assessed as maternal satisfaction with pain relief, visual analogue scales, or use of other pain relief, but meptazinol gave rise to slightly more side effects. Maternal satisfaction with pain relief appeared similar for pentazocine and pethidine, with more frequent nausea and vomiting with pethidine.There is not enough evidence to evaluate the comparative efficacy and safety of the various opioids used for analgesia in labour.