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1. Introduction Analgesics may act at different sites. They may act at the site of injury and decrease the pain associated with an inflammatory reaction, e.g. NSAIDS. They may alter nerve conduction, e.g. local anaesthetics. They may modify transmission in the dorsal horn, e.g. opioids. They may affect the central component and the emotional component of pain, e.g. opioids and anti-depressants. The more sites an analgesic acts at, the better the analgesia. Post-injury analgesia will reduce pain and hypersensitivity for a while, but pain increases again when the analgesic wears off. Pre-emptive analgesia blocks impulses before the injury occurs, therefore preventing hypersensitivity and reducing pain much more effectively.
2. Opioids Opioids can be natural or synthetic drugs, and can be endogenous. Opioid receptors that have been identified include mu, delta, kappa and nociception. Delta and kappa receptors mostly modulate mu receptors. Side effects of opioids include cardiovascular effects, pruritus, urinary retention, vomiting and nausea. a) Morphine Morphine is the most effective opioid at relieving pain. It is a full agonist at mu, delta and kappa receptors. Duration of action is between 4-8 hours. Oral bioavailability is around 25%. Morphine significantly reduces MAC and MIR. It is not licensed for veterinary use and is a schedule II controlled drug. b) Methadone Methadone is a synthetic full mu agonist and has affinity for NMDA receptor. It also acts as a norepinephrine and serotonin re-uptake inhibitor. Following IV administration, duration of action if around 4 hours. Oral bioavailability is poor.
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