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Opioid agonists and partial agonists

Created: 1/6/2004
 Morphine - can be used via the oral, intravenous, intramuscular or subcutaneous route. Slow-release preparations are available. Morphine has significant first-pass (or pre-systemic) metabolism; therefore, the fraction reaching the systemic circulation is much less than that absorbed after oral administration. One of its metabolites, morphine-6-glucuronide, is analgesic in its own right. Morphine induces significant analgesia, but also a host of other effects: respiratory depression, euphoria and sedation, nausea/vomiting, constipation, pupillary constriction (“pin-point” pupil), histamine release (leading to bronchoconstriction and itching).

 Heroin (diamorphine) – is a pro-drug, which is metabolised to morphine (that is ultimately responsible for its effects). It is more lipid soluble than morphine; therefore, the effect after intramuscular administration has a more rapid onset. Its properties make it particularly suitable for epidural administration, to relieve postoperative pain after major surgery. Its higher solubility also constitutes an advantage for continuous subcutaneous infusion.

 Codeine – is an analgesic with  lower efficacy than morphine. Its analgesic effect is due to demethylation in the liver to morphine. It may be used in combination with aspirin or paracetamol and it also has a significant antitussive effect. Like morphine, it induces constipation.

 Pethidine- is a synthetic substance, which is more sedative and has a more rapid onset and a shorter duration of action than morphine. Its metabolite, norpethidine, is active and may accumulate to toxic levels in patients with renal impairment.

 Methadone – is a synthetic compound with a half-life of >24 hours. It leads to a much milder physical abstinence syndrome than morphine but can induce psychological dependence. It is used in maintenance programmes for morphine and heroin addicts.

 Fentanyl – is a highly potent compound, with a half-life of 1-2 hours. It can be used for severe acute pain and during anaesthesia.

 Buprenorphine – is a very lipid soluble compound, which acts as a partial agonist at mu receptors. It is a potent compound but has less efficacy than morphine. Consequently, it may lead to a re-emergence of pain in patients who have received more efficacious opioids, such as morphine. It can be used sublingually and it has a longer duration of action than morphine, but is more emetic. It may induce dysphoria.

Click here for more information about opioids

Opioid antagonists

Naloxone is used in the management of opioid overdose, or to relieve respiratory depression in apnoeic infants after opioids (e.g. pethidine) administered to the mother during labour.

Tolerance and dependence

Tolerance (the necessity to increase the dose in order to achieve the same effect) may develop during chronic administration of drugs, and it may be due to both pharmacokinetic and pharmacodynamic changes. Tolerance to opioids can develop rapidly, especially under experimental conditions. Physical and psychological dependence can also develop. Physical dependence is associated with a withdrawal syndrome when the administration of the drug is stopped abruptly. Psychological dependence leads to craving for the drug.

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