Special Consideration for Using Partial Agonist and Antagonist Opioid Analgesics
27. What are precautions for using partial agonist and antagonist opioids for analgesia in M.T.?
Pentazocine, butorphanol, dezocine, and nalbuphine are partial agonist/antagonist opioid analgesics. Besides exhibiting agonist analgesic effects, they all possess opiate antagonist properties as well. Generally, they act as agonists at κ-opiate receptors and as antagonists at µ-opiate receptors, which explains their ability to simultaneously produce analgesia and precipitate withdrawal symptoms. Pentazocine has an opioid antagonist effect equal to 1/50 of nalorphine; 80 mg has been known to precipitate abstinence in patients who have been given opioids for chronic pain. If a partial opiate agonist is instituted in a patient previously taking a pure agonist, the dose should be increased gradually, and the patient should be observed closely for symptoms of abstinence. If possible, the pure agonist opioid should be withdrawn gradually. If the agonist dose is fairly low and the patient has not been on the drug for >10 days, then the patient can be changed over to pentazocine. In this situation, the patient may experience mild diaphoresis, but should not have significant withdrawal symptoms. A drug-free period of 2 days before the institution of pentazocine also has been suggested, but this is impractical for a patient in pain.
When partial agonist/antagonist opioid analgesics are being considered, the risk of unpleasant psychotomimetic side effects must be weighed against the benefits. For example, butorphanol-induced dysphoric responses are well documented.110,111 A 2-mg dose of butorphanol was associated with an 18% incidence of psychic disturbance, and a 4-mg dose with a 33% incidence. Pentazocine (Talwin) also is associated with a higher incidence of psychotomimetic reactions than traditional opiate agonist analgesics. Hallucinations occurred in 24 of 65 patients (37%) who received 40 to 50 mg of IM pentazocine. In another trial, psychic changes occurred in 1.7% of patients receiving morphine versus 11.4% of patients treated with pentazocine. In comparison, nalbuphine (Nubain) and other analgesics induce much less psychotomimetic effect than butorphanol or pentazocine.112,113 Therefore, a partial agonist/antagonist opioid analgesic should be used in M.T. only if she cannot tolerate the available shorter-acting opioid agonist analgesics.
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