The oral route of administration is the most widely used route and most acceptable for the patient. Disadvantages of the oral route to treat acute pain are that absorption of opioids may be reduced by the delay in gastric emptying that follows surgery. This has the dual disadvantage of non-absorption initially, followed by the possibility of a large dose being absorbed when gastrointestinal function resumes. Thus the oral route may be unsuitable in many instances. 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.