An<unk> is a kind of muscle that acts in opposition to the movement generated by the agonist and is responsible for returning a limb to its initial position.<unk> c muscles are found in pairs called<unk> c pairs. These consist of an extensor muscle, which "opens" the joint (i.e. increasing the angle between the two bones), and a flexor muscle, which does the opposite to an extensor muscle.<unk> c pairs are needed in the body because muscles can only exert a pulling force, and can't push themselves back into their original positions. An example of this kind of muscle pairing is the biceps and triceps.
When the biceps are contracting, the triceps are relaxed, and are able to be stretched back to its original position. This is the opposite when the triceps are contracting.
Antagonist 
Antimetabolite
Anticoagulant
COX-2 inhibitor
Anticholinergic
Antihypertensive
Sympatholytic
Decongestant