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Mixed opioid agonist-antagonists
Mixed opioid agonist-antagonists attempt to relieve pain whilereducing toxic effects and dependency. The mixed opioid agonist-antagonists include:
· pentazocine hydrochloride (combined with pentazocine lactate, naloxone, aspirin, or acetaminophen).
Originally, mixed opioid agonist-antagonists appeared to have less abuse potential than the pure opioid agonists. However, butor-phanol and pentazocine have reportedly caused dependence. Also,patients with chronic pain who are taking an opioid agonist shouldn’t take a mixed opioid agonist-antagonist with it because of the risk of withdrawal symptoms.
Absorption of mixed opioid agonist-antagonists occurs rapidly from parenteral sites. These drugs are distributed to most body tissues and also cross the placental barrier. They’re metabolized in the liver and excreted primarily by the kidneys, although more than 10% of a butorphanol dose and a small amount of a penta-zocine dose are excreted in stool.
The exact mechanism of action of the mixed opioid agonist-antagonists isn’t known. However, researchers believe that these drugs weakly antagonize the effects of morphine, meperidine, and other opiates at one of the opioid receptor sites, while exerting ag-onistic effects at other opioid receptor sites.
Buprenorphine binds with receptors in the CNS, altering percep-tion of and emotional response to pain through an unknown mechanism. It seems to release slowly from binding sites, produc-ing a longer duration of action than the other drugs in this class.
The site of action of butorphanol may be opiate receptors in the limbic system (the part of the brain involved in emotion).
Like pentazocine, butorphanol also acts on pulmonary circula-tion, increasing pulmonary vascular resistance (the resistance in the blood vessels of the lungs that the right ventricle must pump against). Both drugs also increase blood pressure and the work-load of the heart.
Mixed opioid agonist-antagonists are used as analgesia during childbirth and are also administered postoperatively.
Mixed opioid agonist-antagonists are sometimes prescribed in place of opioid agonists because they have a lower risk of drug de-pendence. Mixed opioid agonist-antagonists are also less likely to cause respiratory depression and constipation, although they can produce some adverse reactions. (See Adverse reactions to opioid agonist-antagonists.)
Increased CNS depression and an accompanying decrease in respiratory rate and depth may result if mixed opioid agonist antagonists are administered to patients taking other CNS depressants, such as barbiturates and alcohol
Patients who abuse opioids shouldn’t receive mixed opioid agonist-antagonists because these drugs can cause symptoms of withdrawal.
Mixed opioid agonist-antagonists are listed as pregnancy risk category C drugs; safety and use in breast-feeding women haven’t been established.
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