Which MAT medication is an opioid receptor antagonist?

• A. Acamprosate
• B. Methadone
• C. Buprenorphine
• D. Naltrexone

Answer: (D)

Opioid receptor blockade is achieved by antagonist medications that prevent opioids from producing their effects at the receptors. Naltrexone is the antagonist used in MAT for opioid use disorder. It binds with high affinity to mu-opioid receptors (and also blocks kappa and delta receptors), so opioids cannot elicit euphoria or analgesia even if used. Because it blocks rather than activates, it does not relieve withdrawal and can precipitate it if someone still has opioids in their system, which is why it’s started after detoxification. It’s available as a daily oral form or a long-acting injectable for monthly dosing. It’s not used for replacement therapy; instead, it prevents opioid effects to support abstinence. Acamprosate acts on alcohol-related pathways, not opioid receptors. Methadone is a full mu-opioid receptor agonist, and buprenorphine is a partial mu-opioid receptor agonist with a ceiling effect; both provide opioid effects and help with withdrawal or craving, unlike an antagonist.