Changes

===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. 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.

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===

===Tramadol===

===Tramadol===

Tramadol is an analgesic having weak agonist properties at all opoid receptors but particularly at μ receptors.

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

==Opioid Antagonists==

==Opioid Antagonists==