The sublingual route offers some theoretical advantages for drug administration. Absorption occurs directly into the systemic circulation as there is no first pass metabolism. Tablets can be removed in the event of overdosage and, because of metabolism, they are unlikely to cause toxicity if they are swallowed. It is associated with a high incidence of nausea, vomiting and sedation. Rectal administration .
Most opioid analgesics are subject to extensive metabolism if given by mouth.
The rectal route is a useful alternative, particularly if severe pain is accompanied by nausea and vomiting. Rectal doses for most strong opioids are about half those needed by the oral route. Availability of preparations of opioids for rectal use is very variable throughout the world. Intramuscular administration represents the optimum technique for the developing world where strong opioids are available. As stated previously, this method of analgesia may be associated with peaks and troughs in effect.