Opioid analgesics comprise substances derived from opium (the opiates1) 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 dependence2.
Opioids form a key component in the World Health Organization’s ‘analgesic ladder’ for managing cancer pain.
Opioids are generally full agonists3 at the opioid receptor but some, such as buprenorphine and pentazocine, are mixed agonist–antagonist4 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 serotonergic5 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 abuse6 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.
- A narcotic substance derived from the opium poppy↩
- Dependence is a state that develops as a result of repeated use of a certain chemical substance (eg alcohol, benzodiazepine, opioid, or even medicines for reducing blood pressure). It represents the resetting of homeostasis because of the persisting presence of the chemical substance↩
- A chemical substance that binds to a receptor and mimics the effect of the physiological (endogenous) substance binding to the receptor↩
- A substance that binds to a receptor but produces no effect and inhibits an agonist from binding to the receptor↩
- A chemical agent (or synapse) that produces its effects via the serotonin transmitter system↩
- Any use of an illegal drug or use of a medicine for a non-therapeutic purpose (eg to alter state of consciousness or to seek a ‘high’)↩