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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 agonist/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|morphine]], [[#Methadone|methadone]], [[#Pethidine|pethidine]] or [[#Fentanyl|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; it is an antagnoist at the OP3 receptor and an agonist at the OP2 receptor.

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.

===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 a Schedule 2 controlled drug which is licensed in combination with fluanisone as a neuroleptanalgesic for use in mice, rats, rabbits and guinea pigs.

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===

Etorphine is a hugely potent opioid that is a component of large animal and small animal immobilon. (Large animal immobilon consists of etorphine plus [[Phenothiazines#Acepromazine|acepromazine]]; small animal immobilon comprises etorphine and [[Phenothiazines#Methotrimeprazine|methotrimeprazine]] ). It may be fatal to man follwing self administration, wehn opioid antagonism with [[#Opioid Antagonists|naloxone]] is required.

Etorphine is a hugely potent opioid that is a component of large animal and small animal immobilon. (Large animal immobilon consists of etorphine plus [[Phenothiazines#Acepromazine|acepromazine]]; small animal immobilon comprises etorphine and [[Phenothiazines#Methotrimeprazine|methotrimeprazine]] ). It may be fatal to man following self administration, when opioid antagonism with [[#Opioid Antagonists|naloxone]] is required.

 

Tramadol is an analgesic having weak agonist properties at all opioid receptors but particularly at OP3 receptors.

==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.