Morphine is poorly absorbed from the gastrointestinal tract and is therefore usually administered parenterally. It is metabolised to morphine-6-glucuronide in the liver which is six times more potent than morphine and accumulates in renal failure.

Codeine is a weak analgesic but is favoured by some in head injury since it is less sedative than morphine.

Pethidine has local anaesthetic properties associated with cardiac depression and vasodilatation. It is metabolised to norpethidine which may lead to seizures on accumulation. Respiratory depression occurs despite maintenance of respiratory rate.

Fentanyl and alfentanil are good, short-acting analgesics with poor sedative quality. They cause severe respiratory depression and muscular rigidity. Remifentanil is ultra-short-acting and the patient may suffer from rebound pain if the infusion is stopped temporarily.


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