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Pentazocine provides a type of analgesia that is different from morphine. Its analgesic effect is probably due to an agonist action at K-receptors in the spinal cord; it is a weak antagonist of p-receptors (through which morphine produces analgesia). Thus pentazocine can cause a withdrawal syndrome in addicts (antagonist effect); it can also induce psychological and physical dependence (agonist effect), and this can be severe. It has not proved to be the solution to separating the property of analgesia from that of producing dependence, as was hoped for initially. Its analgesic efficacy approximates to that of morphine, but its potency (weight for weight) is about one-third of morphine. Compared to morphine, pentazocine produces shorter duration of pain relief, less dependence (but this definitely occurs), more psychotomimetic effects, and less sedation and respiratory depression (naloxone can reverse the respiratory depression in overdose).

Pharmacokinetics. Pentazocine is extensively metabolised in the liver and less than 10% is excreted unchanged in the urine (t1/, 5 h).

Uses. Pentazocine is given to relieve moderate to severe pain, and also for chronic pain, for its liability to induce dependence is less than morphine. Its dysphoric effect limits its usefulness.

Adverse effects of this partial agonist include: nausea, vomiting, dizziness, sweating, hypertension, palpitations, tachycardia, central nervous system disturbance (euphoria, dysphoria, psychotomimesis).

Pentazocine has effects on the cardiovascular system, raising systolic blood pressure and pulmonary artery pressure; avoid it in myocardial infarction.

Phenazocine is a high-efficacy agonist used particularly in biliary colic for it has less capacity than other opioids to cause spasm of the sphincter of Oddi. It may be administered sublingually if the patient is vomiting.

Buprenorphine is a high-efficacy partial agonist of the p-receptor and an antagonist of the K-receptor. Its high receptor affinity (tenacity of binding) may explain why respiratory depression is only partially reversed by naloxone; a respiratory stimulant (doxapram) may be needed in overdose, or mechanical ventilation. It has less liability to induce dependence and respiratory depression than pure agonists, little effect on the cardiovascular system and may spare the sphincter of Oddi from induced spasm. Its t1/^ is 5 h. Because of extensive presystemic elimination when swallowed, buprenorphine is given by the buccal (sublingual) route (200-400 micrograms) or by i.m., or slow i.v., injection (300-600 micrograms). It is a useful analgesic because of the length (about 6 h) and strength of its action, its low liability to cause dependence and the fact that administration by injection can be avoided, e.g. for children, patients with bleeding disorder.

Dextropropoxyphene is structurally similar to methadone and differs in that it is less analgesic, antitussive, and less dependence-producing. Its analgesic usefulness approximates to that of codeine. Dextropropoxyphene is rapidly absorbed from the gastrointestinal tract and its plasma t1/ is 5 h. In overdose the rapidity of absorption is such that respiratory arrest may occur within one hour and also hypotension (probably due to membranestabilising or quinidine-like action causing cardiac arrhythmia), so that many subjects die before reaching hospital. Combination with alcohol (common with self-poisoning) enhances respiratory depression. Dextropropoxyphene is commonly combined with paracetamol (co-proxamol, Distalgesic). Dextropropoxyphene interacts with warfarin, enhancing its anticoagulant effect.

Dihydrocodeine (DFI18) is a low-efficacy opioid with an analgesic efficacy similar to that of codeine. It is used to relieve moderate acute and chronic pain on its own or as a compound tablet (co-dydramol; dihydrocodeine 10 mg plus paracetamol 500 mg). Dihydrocodeine causes histamine release and should not be used in patients with hyper-reactive airways.

Meptazinol is a high-efficacy partial agonist; it also has central cholinergic activity which add to its analgesic effect. It is used to relieve acute or chronic pain of moderate intensity, e.g. postoperatively and in obstetrics. Meptazinol does not cause euphoria and withdrawal effects seem not to occur when it is discontinued. It appears not to induce a withdrawal syndrome in opioid-dependent subjets.

Tramadol is an opioid with additional actions; the basis of its analgesic effects appears to derive from a combination of (relatively weak) agonist action on p-receptors, inhibition of neuronal noradrenaline uptake and enhanced serotonin release. It is rapidly absorbed from the gastrointestinal tract, 20% of an oral dose undergoes first-pass metabolism and less than 30% dose is excreted unchanged in the urine (t'/2 6 h). Tramadol is approximately as effective as pethidine for postoperative pain and as morphine for moderate chronic pain.

Tramadol is claimed to be less likely to constipate, depress respiration and addict. Confusion, convulsions, hallucinations and anaplhylaxis have been reported with its use.

Dipipanone is less sedating and shorter acting than morphine; it is suitable for acute attacks of pain, e.g. breakthrough pain in terminal illness (Diconal is dipipanone plus cyclizine, an antiemetic).

Dextromethorphan, the dextroisomer of the opioid levomethorphan is used as an antitussive, e.g. in Actifed; the latter is sought as a drug of abuse by addicts.

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