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Opioids are chemicals that bind to the opioid receptors, which are mainly located in the central nervous system and gastro-intestinal tract.

Broadly there are four groups of opioids:

  1. Endogenous opioid peptides, produced in the body, including endorphins, enkephalins and dynorphins;
  2. Opium alkaloids, such as morphine (the prototypical opioid) and codeine phosphate which 85% of the population metabolise to morphine;
  3. Semi-synthetic opioids such as heroin and oxycodone;
  4. Fully synthetic opioids such as pethidine and methadone that have structures unrelated to the opium alkaloids.

The term opiate is often used interchangeably with opioid, but it is more correctly applied only to the natural opium alkaloids and their semi-synthetic derivatives. Toxicological analysis can distinguish natural from synthetic opioids by detection of compounds such as noscapine which are found only in opiates (i.e. poppy-derived).



Clinical Use


While used mainly as analgesics they have also been used as sedatives (eg in psychoses before more effective agents were developed)

Clinical Differences

Can be quite marked, with some having partial agonist and antagonist properties. For example pentazocine, butorphanol and nalbuphine are weak mu-antagonists and kappa-partial-agonists and only the first has any real clinical use. By injection pentazocine is one-sixth to one-third as potent as morphine, nalbuphine is slightly less potent than morphine, and butorphanol is several times as potent and the later two are belived to have less abuse potential. Pentazocine, by mouth, is a weak analgesic, more similar in efficacy to aspirin and paracetamol than other weak opioids like codeine and has psychotomimetic side effects in up to 20% of patients. Buprenorphine is a potent mu-receptor partial agonist. It is 30 times as potent as morphine but may have a ceiling to the analgesic effect. Despite its limited effective dose range and the same side effects as other morphine-like drugs, possibly more frequently at equianalgesic doses, its long half life can be of advantage in chronic pain control. In acute analgesia butorphanol, nalbuphine and buprenorphine, have largely replaced pentazocine as they cause a low incidence of dysphoric side effects and their degree of respiratory depression is not dose related with a ceiling effect as the dose is increased. Nalbuphine can reverse morphine-induced respiratory depression while maintaining adequate analgesia. Butorphanol increases heart rate, arterial and pulmonary blood pressures and cardiac output but therefore increases risk in those with pre-existing ischaemic heart disease. Pethidine which is widely used in low dose as a supplement to nitrous oxide in cardiac and noncardiac surgery, is unsuitable because of severe hemodynamic disturbances at high dose. Fentanyl, as one of the potent opioid agonists is extensively used as a supplement to general anaesthesia. Alfentanil with its very short elimination half-life is used during short operations and for day-case surgery, so the problem of respiratory depression common to all strong opiate agonists is minimised during recovery. It also has more intense vagomimetic properties than fentanyl. Sufentanil is about 10 times more potent than fentanyl, is more lipophilic and is more rapidly eliminated. Remifentanil is rapidly metabolised by non-specific esterases producing a rapid and predictable postoperative awakening, even after long procedures. However its marked vagomimetic actions necessitates atropine pre-medication especially in small children, the elderly and hypovolaemic patients.

Clinical Issues


Group Contra-indications

Group Cautions and Interactions

Group Side effects

  • Nausea and vomiting
  • Constipation
  • Drowsiness
  • Respiratory depression
  • Dependence
  • Hallucinations & Confusion

Special advice

Know several and be wary whenever a new one enters the market with claimed lower abuse potential




This category has the following 6 subcategories, out of 6 total.






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