Major Classes of Opioid Analgesics in Clinical Practice

Among the commonly known classes of opioids/opiates being used in practice are morphine, codeine, heroin, and the antagonist naloxone (Figure II-6). Morphine by itself is still made from opium and although there is a major first-pass effect (i.e. degradation by liver enzymes), oral administration is still possible, but requires substantial dosage increase. Codeine, which is also taken orally, has a strong ability to inhibit coughing, but it induces less analgesia. Among the phenylpiperidines a number of synthetic compounds have entered the market. The most known is meperidine/pethidine (Demerol®), which is very similar to morphine, but is more efficacious when given orally for the control of pain. Another derivative is loperamide (Imodium®), an agent being used as a common antidiarrheal, which does not enter the brain, as it is incapable of crossing the blood-brain barrier. Hence it is not abused and therefore is sold as a DOC (drug over the counter). Contrary, fentanyl (Sublimaze®) is an opioid, which is at least 200 times as potent as morphine. This agent is used with nitrous oxide or droperidol (a neuroleptic) in intravenous anesthesia (neuroleptanesthesia), but it is also a used in a transdermal patch for the control of chronic pain. Another known opioid is methadone, which has a good oral efficacy, a much longer half-life than morphine (8-12 h), and in regard to its effect much like morphine. It is used for treatment of heroin addiction and for the control of chronic pain. A methadone congener, which is being used solely in the methadone substitution programs is LAAM (a-levoacetylmethadol), only needs to be taken once every 72 h. The opioid propoxyphene (Darvon®) has

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Figure II-6. Molecular structure of different opioid ligands with agonistic or antagonistic properties the lowest analgesic potency (0.02 times morphine). It is almost always given together with aspirin for the control of mild to moderate pain. It is very popular clinically due to misplaced concerns about the abuse potential of codeine.

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