What is an antagonist in medicine?

In literature, an antagonist is a character, group of characters, or an institution which opposes the protagonist, the main character(s). The antagonist struggles against, opposes, or competes with the protagonist.

In biochemistry, the antagonist is a substance that interferes with or inhibits the physiological action of another.

In anatomy, the antagonist is a muscle whose action counteracts that of another specified muscle.

In pharmacology, the antagonist is a drug that counteracts the effects of another drug.