Small doses of opioids given with induction can usefully reduce the dose requirements of drugs used during anaesthesia. Those used are:
Fentanyl (t1/, 3 h) has higher efficacy than morphine, analgesia lasts 30-60 min (single dose) and is used i.v. Fentanyl is also given for chronic and intractable cancer pain as self-adhesive patches which release the drug at approximately 25 micrograms /h for 72 h. Fentanyl is so potent that discarded patches may yet contain sufficient drug to be dangerous.
Alfenatil (t1/ 1.5 h), given i.v., provides maximum analgesia in 90 seconds, which lasts about 5-10 min from a single dose; it is used for brief (painful) operations.
Remifentanil is rapidly metabolised, not in the liver but by blood and tissue esterases. Its short duration of action renders it well suited for continuous i.v. infusion without accumulation.
Opioids (nonanalgesic) for an antimotility effect on the gut include loperamide and diphenoxylate (p. 644).
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