A Cochrane review included 54 studies with a total of 3749 patients. The comparisons were either between modified release morphine and immediate release morphine, between different strengths, dose intervals or administration routes, or between morphine and other opioids. There were no placebo-controlled studies. Daily doses in studies ranged from 25 mg to 2000 mg with an average of between 100 mg and 250 mg. Morphine was shown to be an effective analgesic. Pain relief did not differ between sustained release and immediate release preparations. Sustained release versions of morphine were effective for 12 or 24 hour dosing depending on the formulation. Dose titration were undertaken with both forms. Adverse effects were common but only 4% of patients discontinued treatment because of intolerable adverse effects. There is limited evidence to suggest that transmucosal fentanyl provides more rapid pain relief for breakthrough pain compared to morphine.