Of the currently available mu agonist drugs, the following are relatively contraindicated: 1. Methadone--unpredictable duration of action [5]. 2. Pethidine--unwanted central effects, metabolised to an active metabolite and too short acting. 3. Codeine--too weak and with constipating side-effects. 4. Fentanyl--too short acting. 5. Oxycodone--too short acting although suppositories may overcome some theoretical disadvantages. 6. Dextropropoxyphene--weak agonist which is possibly metabolised to a cardiotoxic metabolite [6]. Morphine remains the drug of choice for chronic pain when administered in a sustained release preparation. MS Contin, a slow release oral formulation of morphine, is available and has a predictable duration of action lasting from 8-12 h, while improved formulations are about to be released in the near future in some countries. Prescribers need to take into account the relatively poor oral bioavailability of morphine when calculating the daily morphine dose.
Analgesics Chronic Disease