An agonist is a substance that mimics the actions of a neurotransmitter or hormone to produce a response when it binds to a specific receptor in the brain. Opioid drugs, for example heroin and methadone, are agonists that produce responses such as ‘liking’, analgesia and respiratory depression.

In contrast to the action of an agonist, an antagonist, such as naltrexone, binds to a specific receptor in the brain but does not activate it. Therefore, if an agonist, for example heroin or methadone, is present and activating the receptor, taking naltrexone will counteract the activation, resulting in withdrawal.

Agonist: A chemical substance that binds to and activates certain receptors on cells, causing a biological response. Oxycodone, morphine, heroin, fentanyl, methadone, and endorphins are all examples of opioid receptor agonists. Antagonist: A chemical substance that binds to and blocks the activation of certain receptors on cells, preventing a biological response. Naloxone is an example of an opioid receptor antagonist (NIDA glossary).