The opioids discussed below are considered in relation to morphine. Note that the t/ does not necessarily indicate duration of useful analgesia, which is also related to affinity of the opioid for receptors; but t1/, gives useful information on accumulation.
Codeine is a low-efficacy opioid that binds to p-receptors; 10% is converted to morphine (tV2 3 h). It lacks efficacy for severe pain and most of its actions are about one-tenth those of morphine. A qualitative difference from morphine is that large doses cause excitement. Dependence occurs but much less than with morphine.
Its principal uses are for mild and moderate pain and cough (long-term use is accompanied by chronic constipation) and for the short-term symptomatic control of the milder acute diarrhoeas. There are numerous formulations for cough, e.g. Codeine Linctus, and for pain, in which it is commonly combined with paracetamol and/or aspirin.
Pethidine attracted attention as a possible analgesic because it caused the tails of laboratory mice to stand erect (Straub phenomenon), a characteristic of morphine-like drugs caused by spasm of the anal sphincter.
Pethidine binds to the p- and k-receptors; it is effective for moderate or severe pain but its duration of action is shorter than that of morphine. It is effective against pain beyond the reach of codeine. Despite its substantial structural dissimilarity to morphine, pethidine has many similar properties including that of being antagonised by naloxone.
Pethidine differs from morphine in that it:
• does not usefully suppress cough
• is less likely to constipate; but its effect in the upper small intestine is similar to morphine including contraction of the sphincter of Oddi
• is less likely to cause urinary retention and to prolong childbirth
• has little hypnotic effect
Pethidine is extensively metabolised in the liver and the parent drug and metabolites are excreted in the urine (tV2 5 h). Norpethidine retains pharmacological activity and may accumulate dangerously when renal function is impaired.
Pethidine causes vomiting about as often as does morphine; it has atropine-like effects, including dry mouth and blurred vision (cycloplegia and sometimes mydriasis, though usually miosis). Overdose or use in renal failure can cause central nervous system stimulation (myoclonus, convulsions) due to norpethidine.
There is disagreement on the extent to which pethidine depresses respiration. It is probable that in equianalgesic doses it is as depressant as morphine.
Pethidine dependence occurs, with some tolerance, especially to the side-effects, but its psychic effects are less constant and less marked than those of morphine. Pethidine has evident advantages over morphine for pain that is not very intense, and it is widely used. It is usually given orally (50-100 mg) s.c. or i.m. (25-100 mg), when its effects last 2-3 h. It is widely used in obstetrics because it does not delay labour like morphine; but it enters the fetus and can depress respiration at birth.
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