Histamine release by morphine and diamorphine in man.

Intravenous morphine and diamorphine are routinely used for postoperative analgesia but the relative histamine releasing abilities of these drugs have not been compared in man. Thirty-eight patients were randomly allocated to receive morphine (0.16 mg.kg-1) or diamorphine (0.08 mg.kg-1) after abdominal surgery. Blood samples for histamine were taken before, and at timed intervals after, opioid administration and analysed by an isotopic radioenzymatic technique. Haemodynamic parameters and pain scores were recorded before and after analgesic administration, and a series of eight basophil histamine release studies was also performed. Significant histamine release (plasma concentration > 2 ng.ml-1 or rise of > 700% baseline) occurred in 23.5% of the morphine group and 21.1% of the diamorphine group. Histamine was released earlier in those receiving diamorphine, but no significant change in haemodynamic parameters occurred, and no histamine release was demonstrated in the basophil histamine release studies. These findings suggest that morphine and diamorphine release histamine from mast cells rather than basophils.