Mixed opioid agonist/antagonist used for mild to moderate pain; 4 to 7 times more powerful than morphine; common component of preanesthetic protocols. also used as an antitussive; class IV controlled substance

• A. Morphine
• B. Fentanyl
• C. Butorphanol
• D. Buprenorphine

Answer: (C)

Butorphanol is a mixed opioid agonist/antagonist with kappa receptor agonist activity and mu receptor antagonism (or partial agonism). This combination provides analgesia for mild to moderate pain while producing less respiratory depression than pure mu agonists, and it often has a ceiling effect on respiratory depression, which helps make it safer in perioperative settings. Its potency relative to morphine—about four to seven times more powerful—means effective pain relief can be achieved at lower doses. It also has antitussive properties, so it’s commonly used to suppress coughs in small animals and is a familiar component of preanesthetic protocols due to its analgesic and sedative effects with a lower risk of severe respiratory depression. It’s classified as a schedule IV controlled substance because of potential for abuse, but generally has a lower abuse risk than full mu agonists.

Morphine is a pure mu agonist with high analgesic potency but greater risk of respiratory depression and no mu antagonism. Fentanyl is another pure mu agonist with very high potency and rapid onset, not the mixed agonist/antagonist profile described. Buprenorphine is a partial mu agonist (with high receptor affinity and long duration) and, although also used in analgesia, does not fit the described four-to-sevenfold potency and antitussive usage as cleanly as butorphanol.