An agonist is a drug that activates certain receptors
in the brain. Full agonist opioids activate the opioid receptors
in the brain fully resulting
in the full opioid effect. Examples of full agonists are heroin, oxycodone, methadone, hydrocodone, morphine, opium and others.
An antagonist is a drug that blocks opioids by attaching to the opioid receptors without activating them. Antagonists cause no opioid effect and block full agonist opioids. Examples are naltrexone and naloxone.