Alfentanil: a study of its analgesic activity and interactions with morphine in the mouse.

Alfentanil has a short duration action which did not markedly increase with increasing dose in mice. It is approximately one-fourth as potent as fentanyl. Repeat injections gave reproducible analgesic effects but depression of respiratory rate increased when repeat injections were given on the basis of cessation of analgesic activity. Alfentanil appeared to reduce the analgesic effect of morphine given 30-60 min after alfentanil administration. This has been demonstrated with injection of alfentanil 15 min before, simultaneously with and 30 min after morphine injection. There were not significant effects on depression of respiratory rate produce by morphine.