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The opioid drugs include [[#Morphine|morphine]] and its synthetic derivatives. They are powerful '''analgesics''', and have dose-dependent '''sedative''' properties.

The opioid drugs include [[#Morphine|morphine]] and its synthetic derivatives. They are powerful '''analgesics''', and have dose-dependent '''sedative''' properties.

===Buprenorphine===

===Buprenorphine===

Buprenorphine is a Schedule 3 controlled drug used for the relief of moderate pain. It is licensed for veterinary used and may be given i/v, i/m, s/c or p/o. Its actions last 6-8 hours.

Buprenorphine is a Schedule 3 controlled drug used for the relief of moderate pain. It is licensed for veterinary used and may be given i/v, i/m, s/c or p/o. There is an onset time of 45 mins and its actions last 6-8 hours.

Buprenorphine is a partial OP3 agonisy/antagonist. This gives it a bell-shaped dose response curve, with large experimental doses actually causing opioid receptor antagonism. Buprenophine may therefore cause partial reversal of administration of full OP3 agonists such as morphine, methadone, pethidine or fentanyl.

Buprenorphine is a partial OP3 agonist. This gives it a bell-shaped dose response curve, with large experimental doses actually causing opioid receptor antagonism. Buprenophine may therefore cause partial reversal of administration of full OP3 agonists such as [[#Morphine|morphine]], [[#Methadone|methadone]], [[#Pethidine|pethidine]] or [[#Fentanyl|fentanyl]].

===Butorphanol===

===Butorphanol===

Butorphanol may be used to combat mild pain. It is a poor analgesic but has excellent sedative properties, and also acts as a cough suppressant. Butorphanol is licensed for veterinary use, and is a mixed partial agonist.

Butorphanol may be used to combat mild pain. It is a poor analgesic but has excellent sedative properties, and also acts as a cough suppressant. Butorphanol is licensed for veterinary use, and is a mixed partial agonist; it is an antagonist at the OP3 receptor and an agonist at the OP2 receptor.

There is an onset time of 15 mins following i/v, i/m or s/c administration, and effects then last for 2-4 hours.

There is an onset time of 15 mins following i/v, i/m or s/c administration, and effects then last for 2-4 hours.

===Pethidine===

===Pethidine===

A full OP3 agonist, pethidine is used for moderate to severe pain. It may be given i/m or s/c, but NOT intravenously as it will cause massive histamine release. The onset time is 10-15 mins, and the drug has as duration of 30-60 mins. Pethidine is chronotropic and also spasmolytic. It is therefore a good drug choice to provide analgesia and facilitate rectal examination of colic-ing horses. Pethidine is a Schedule 2 controlled drug.

A full OP3 agonist, pethidine is used for moderate to severe pain. It may be given i/m or s/c, but NOT intravenously as it will cause massive histamine release. The onset time is 10-15 mins, and the drug has as duration of 30-60 mins. Pethidine is chronotropic and also spasmolytic. It is therefore a good drug choice to provide analgesia and facilitate rectal examination of colic-ing horses. Pethidine is a licensed, Schedule 2 controlled drug.

===Methadone===

===Methadone===

Methadone is an unlicensed, Schedule 2 controlled, full OP3 agonist. It is used i/v, i/m, s/c or by constant rate infusion for the relief of moderate to severe pain. Advantageously, it does not cause emesis.

Methadone is licensed for use in dogs and cats in the UK.  It is a Schedule 2 controlled drug, and a full OP3 agonist. It is used i/v, i/m, s/c.  It accumulates in the body over time, so is not recommended as a constant rate infusion.  It is used for the relief of moderate to severe pain. It is considered equipotent to morphine in its activity at the OP3 receptor, but also acts as an NMDA receptor antagonist, theoretically enhancing its analgesic efficacy.  Advantageously, it rarely causes emesis. There is an onset time of 15-30 mins following i/v, i/m or s/c administration, and effects then last for around 4 hours.

===Fentanyl===

===Fentanyl===

Fentanyl may be used i/v or via slow release patches for severe pain. It is a full OP3 agonist with a rapid onset of action and a duration of 15-20 mins. It is therefore useful as "rescue analgesia" to top-up levels of analgesia during particularly painful stages of surgery. It is an unlicensed, Schedule 2 controlled drug.

Fentanyl may be used i/v or via slow release patches for severe pain. It is a full OP3 agonist with a rapid onset of action and a duration of 15-20 mins. It is therefore useful as "rescue analgesia" to top-up levels of analgesia during particularly painful stages of surgery. It is a Schedule 2 controlled drug which is licensed as a formulation containing fluanisone as a neuroleptanalgesic for use in mice, rats, rabbits and guinea pigs.

===Etorphine===

===Etorphine===

==Opioid Antagonists==

==Opioid Antagonists==

Naloxone is an antagonist to endogenous opioids. It is frequently used in human medicine to antagonise opioid narcotics in the incidence of overdose. Naloxone may be administered intravenously, intramuscularly, intrathecally or subcutaneously. It has a short duration of action (1-2 hours) and so repeated administrations may be necessary.

Naloxone is an antagonist to endogenous opioids. It is frequently used in human medicine to antagonise opioid narcotics in the incidence of overdose. Naloxone may be administered intravenously, intramuscularly, intrathecally or subcutaneously. It has a short duration of action (1-2 hours) and so repeated administrations may be necessary.