The clinical usefulness of agonist-antagonist analgesics in acute pain.

The opioid agonist-antagonists are a heterogeneous group of compounds capable of providing analgesia sufficient to treat moderate to severe acute pain. Pentazocine, butorphanol and nalbuphine produce subjective effects which are quite different from those of morphine. Lack of mood elevation and occasional dysphoria may contribute to a lower level of patient acceptance, but all of these analgesics are significantly safer than the pure agonists. Doses in the therapeutic range are unlikely to produce dangerous levels of respiratory depression in most patients. Other opioid side-effects such as nausea, constipation and biliary spasm appear to be less frequent as well. The mu partial agonist buprenorphine shares many of the safety advantages of the older drugs, and its subjective effects appear more morphine-like. It is not clear whether mu partial agonists have real clinical advantages over kappa-type analgesics. All of these drugs are opioid antagonists and are able to precipitate abstinence in individuals with significant prior exposure to opiates. Neither absolute potency nor the ratio of agonist to antagonist effect are predictors of therapeutic usefulness. There is now an enormous amount of clinical experience with the agonist-antagonists. In many, but not all, clinical situations they are acceptable alternatives to the morphine-like drugs.