In pharmacology the term agonist–antagonist is used to refer to a drug which exhibits some properties of an agonist (a substance that fully activates the neuronal receptor that it attaches to) and some properties of an antagonist (a substance that attaches to a receptor but does not activate it or if it displaces an agonist at that receptor it seemingly deactivates it thereby reversing the effect of the agonist).
The best known agonist–antagonists are opioids. Examples of such opioids are:
Agonist–antagonist opioids usually have a ceiling effect – over particular dose they don't increase their potency.
Some preparations of buprenorphine (e.g. Suboxone, in contrast to Subutex) contain naloxone in order to deter intravenous injection (oral naloxone is almost inactive) – it is not related to agonist–antagonist concept.