- Butorphanol is classified as a mixed agonist-antagonist opioid. This means that it does not act purely as an agonist or antagonist at all opioid receptor sites but instead has different actions depending on the receptor subtype.
- Specifically, butorphanol acts as a partial agonist at kappa (κ) opioid receptors and as an antagonist or partial agonist at mu (µ) opioid receptors.

- The κ-agonist activity contributes to its analgesic effects, especially in the spinal cord and brain, while its µ-opioid receptor antagonism or partial agonism limits some of the typical side effects seen with pure µ-agonists, such as respiratory depression and euphoria.
- This property also reduces the potential for addiction and abuse relative to pure µ-agonists like morphine.

- Due to this mixed profile, butorphanol is used clinically for moderate to severe pain and in some cases for anesthesia adjuncts. Its pharmacological profile also explains why it can precipitate withdrawal symptoms in patients dependent on pure µ-opioid agonists.

In summary, butorphanol's primary mode of action is best described as a mixed κ-agonist/µ-antagonist opioid receptor modulator, rather than a pure µ-opioid receptor agonist or antagonist.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 14: Opioid Analgesics, pp. 550-555.