This systematic review of single-dose, placebo-controlled, randomized trials assessed pain relief from subcutaneous, intramuscular or intravenous morphine compared with placebo in postoperative pain. Pain relief or pain intensity difference over 4 to 6 hours was extracted and converted into the number of patients with at least 50% pain relief. This was used to calculate the relative benefit and the number-needed-to-treat (NNT) for one patient to achieve at least 50% pain relief. In 15 trials, comparing intramuscular morphine 10 mg (486 patients) with placebo (460 patients) morphine had an NNT of 2.9 (95% confidence interval 2.6-3.6). This meant that one of every three patients with moderate or severe postoperative pain treated with 10 mg intramuscular morphine had at least 50% pain relief, and would not have achieved this had they been given placebo. Minor adverse effects were more common with morphine (34%) than with placebo (23%) (relative risk 1.49 [1.09-2.04]), but drug related study withdrawal was rare (1.2% overall) and no different from placebo.