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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.

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