Which opioid is used to treat colic pain in horses and is a Class II controlled substance?

• A. Meperidine
• B. Morphine
• C. Methadone
• D. Buprenorphine

Answer: (C)

In horses with colic, strong, reliably acting opioid analgesia is needed for visceral pain, and methadone fits well here. It’s a synthetic mu-opioid receptor agonist with high potency and a relatively long duration, providing effective relief of abdominal pain. Its analgesic effects are supported by additional action on NMDA receptors, which can help with pain that becomes persistent or tolerant to other opioids. Importantly, methadone tends to cause less histamine release than morphine, reducing the risk of hypotension in a stressed colic patient. It is a Class II (Schedule II) controlled substance, meaning it has accepted medical use but is tightly regulated due to abuse potential. Other opioids listed have drawbacks for acute colic: meperidine can produce neurotoxic metabolites and is less favored; morphine can cause significant histamine release and hypotension; buprenorphine is a partial agonist with a ceiling effect and is scheduled as III, not II.