ABSTRACT
All clinically useful opioids are µ agonists, but use has been made of partial agonists such as buprenorphine and nalbuphine. Buprenorphine is a partial µ agonist, and nalbuphine a partial κ agonist which has antagonistic actions at the µ receptor. Tramadol is a µ, κ and δ agonist which stimulates the release of serotonin and inhibits the reuptake of noradrenaline. It is useful to consider opioids as weak or strong depending on their relative efficacy. Codeine is a weak opioid, whose effects are maximum at about 200 mg per day. Strong opioids include morphine, its prodrug diamorphine, fentanyl, oxycodone, hydromorphone, methadone and buprenorphine. Transdermal preparations of fentanyl and buprenorphine are now available and marketing authorization has been given for their use in non-cancer pain. If one opioid is not effective, an alternative may be tried. The practice of ‘opioid rotation’, pioneered in the management of cancer pain, is worth trying in chronic non-cancer pain. The logic of this technique, in which different opioids are used to maintain analgesia while minimizing side effects, is attributed to incomplete cross-tolerance between different opioids.
