In anatomy, an antagonist is a muscle (see skeletal muscles and muscle groups) that acts in opposition to another muscle. For example, the biceps and triceps are an antagonistic pair.

 

In medicine, an antagonist is a drug or hormone that acts to oppose the action of another drug or hormone.

 

In each case, the opposing muscle, drug, or hormone is called the agonist. The antagonist to a drug is sometimes called an antidote.