Butorphanol

Butorphanol is an agonist at κ-receptors. Its activity at μ-receptors is either antagonistic or partially agonistic. It is five to eight times as potent as morphine and is only available in parenteral form. After IM injection, onset of effect is rapid, and peak analgesia occurs within 1 hour. Whereas duration of action of butorphanol is similar to that of morphine, its plasma half-life is only 2 to 3 hours. Although butorphanol (10 mg IM) causes as much respiratory depression as the same dose of morphine, higher doses reach a ceiling. Side effects after butorphanol include drowsiness, sweating, nausea and CNS stimulation.

In healthy volunteers, butorphanol (0.03 or 0.06 mg/kg IV) produces minimal cardiovascular changes. However, in patients with cardiac disease, butorphanol causes significant increases in cardiac index, left ventricular end-diastolic pressure and pulmonary artery pressure.

Because butorphanol decreases the MAC for enflurane by only a small fraction, it cannot serve like the fentanyl congeners as an anesthetic agent. Butorphanol does partially antagonize fentanyl-induced respiratory depression.^[477] It is subject to less abuse and has less addictive potential than morphine or fentanyl. Acute biliary spasm can occur after butorphanol, but increases in biliary pressure are less than after equipotent doses of fentanyl or morphine.