Structurally, butorphanol is a morphinan and shares the same cyclobutyl methyl group on the nitrogen as nalbuphine. Like nalbuphine, butorphanol is an agonist at the K-receptor but at the /-receptor butorphanol is both a partial agonist and an antagonist.111 The affinity for opioid receptors in vitro is 1:4:25 for the /-, 8-, and K-receptors respectively.111 The high affinity for the K-receptors is proposed to give butor-phanol its analgesic properties and is also responsible for the CNS adverse effects such as hallucinations, psychosis, and paranoid reactions. Butorphanol binds with /-receptors as a partial agonist, and administration to humans maintained on high-potency /-agonists such as morphine may precipitate withdrawal. Butorphanol was found to produce convulsions in morphine-deprived, morphine-dependent monkeys.112

The parenteral injection is used for moderate to severe pain associated with orthopedic procedures, obstetric surgery, and burns. The recommended dose is 1 mg IV or 2 mg intramuscular (IM) every 3 to 4 hours as needed. Early studies proposed a "ceiling effect" for butorphanol's antinociception and respiratory depressant effect. More recently, the WHO Expert Committee on Drug Dependence performed a critical review of butorphanol and found no ceiling effect to the respiratory depressant effect of parenteral butorphanol in monkeys and humans.112 The nasal preparation (Stadol NS) is an effective analgesic for the relief of moderate to severe pain such as migraine attacks, dental, or other surgical pain where an opioid is appropriate. The nasal spray is administered 1 mg (1 spray in one nostril) with an additional spray 60 to 90 minutes later if adequate pain relief is not achieved. Respiratory depression is not an issue with normal doses of the nasal preparation, but CNS side effects are the same as parenteral butorphanol. Increased reports of abuse and addiction of the nasal spray led the FDA to change the product to a Schedule IV drug in 1997.


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