Opioid analgesics comprise substances derived from opium (the opiates) as well as synthetic substances that act on opioid receptors in the body. Clinically, opioids are used mainly as analgesics but they can also be used for their cough-suppressant effect and to reduce intestinal motility. Some opioids are used in the management of opioid dependence.
Opioids form a key component in the World Health Organization’s ‘analgesic ladder’ for managing cancer pain.
Opioids are generally full agonists at the opioid receptor but some, such as buprenorphine and pentazocine, are mixed agonist–antagonist compounds. In addition to opioid agonist effect, tapentadol has noradrenergic activity which contributes to its analgesic effect. Tramadol is an opioid analgesic with noradrenergic and serotonergic properties which may also contribute to the analgesic effect.
The opioids diphenoxylate (combined with atropine as co-phenotrope) and loperamide are used for managing diarrhoea only; both are derived from pethidine. Loperamide is metabolised in the liver and very little of the intact drug enters the general circulation. Diphenoxylate has no analgesic effect; atropine is included in the formulation to deter abuse of diphenoxylate. Diphenoxylate and loperamide are not considered in the discussion of opioid risks, below.
In England, over 21.5 million primary-care prescriptions were dispensed for opioid analgesics in 2013; over 35% of these were for tramadol. A further 3.4 million prescriptions were for opioids licensed specifically for managing opioid dependency.
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