Methadone is an opioid: antagonist or agonist?

• A. Antagonist
• B. Agonist
• C. Partial agonist
• D. Mixed agonist-antagonist

Answer: (B)

Methadone acts as a full agonist at the mu-opioid receptor. As a full agonist, it activates the receptor to its maximal effect, producing analgesia, euphoria, and, importantly for opioid use disorder, suppression of withdrawal and cravings. Its long half-life makes it useful for maintenance therapy, but it also means dosing must be carefully managed to avoid accumulation and respiratory depression.

It is not an antagonist, which would block the receptor and prevent opioid effects (and could precipitate withdrawal if given to someone dependent). It is not a partial agonist, which would produce only a partial maximal effect (examples include buprenorphine, which has a ceiling effect for respiratory depression). It is not a mixed agonist-antagonist, which would act as an agonist at some receptors and an antagonist at mu (such as pentazocine).