Wikipedia
Agonist-antagonist
In pharmacology the term agonist–antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist (a substance that fully activates the receptor that it binds to) while under other conditions, behaves as an antagonist (a substance that binds to a receptor but does not activate and can block the activity of other agonists).
Types of mixed agonist/antagonist include receptor ligands that act as agonist for some receptor types and antagonist for others or agonist in some tissues while antagonist in others (also known as selective receptor modulators).