Main Difference – Agonist vs Antagonist
Agonist and Antagonist are two contradictory terms, i.e., they are antonyms. These two terms also have different meanings in different fields. The terms, agonists and antagonists are mainly found in the fields of anatomy, biochemistry and literature. As mentioned above, agonist and antagonist act in opposite ways; when an agonist produces an action, antagonist produces its opposite action. This is the main difference between agonist and antagonist.
Agonist – Meaning and Usage
As mentioned earlier, agonist has different meanings. When we are talking about the human body, however, agonist is a muscle. This muscle’s contraction helps to move a part of the body directly. In biochemistry, agonist is a substance which initiates a physiological response when combined with a receptor. In pharmacology, agonist is a drug which imitates the action of neurotransmitters in the brain.
In literature, agonist is the equivalent of a protagonist. Agonist refers to the leading character of a book, play, drama, etc. For example, Harry Potter is the agonist of the Harry Potter series.
Oliver is the protagonist (agonist) in Charles Dicken’s Oliver Twist.
Antagonist – Meaning and Usage
Antagonist too has many different meanings, just like agonist. But what is interesting about this term is that it has the exact opposite meanings from agonist.
In anatomy, antagonist is a muscle whose action counteracts that of an agonist muscle. Agonist and Antagonist muscles usually occur in pairs; when one muscle relaxes, the other contracts. Biceps and triceps are an example of this kind of muscle pair.
Since the chemical agonist instigates a physiological response when combined with a receptor, antagonist does the exact opposite; antagonist inhibits or interferes with the physiological response of agonist.
As a drug, antagonists block the neurotransmitters in the brain. This is in contrast to the action of agonists: mimicking the effects of neurotransmitters.
As seen from all the above meanings, it becomes clear that agonists produce an action, or a reaction and antagonists produce its opposite action.
Antagonist has another meaning, which is perhaps the most common meaning of this word. An antagonist is a person who opposes something or is hostile to something or someone. In literature, antagonist is the main opposing force of the protagonist. 
Difference Between Agonist and Antagonist
Opposition
Agonist produces an action or response.
Antagonist produces its opposite action or response.
Muscle
Agonist is a muscle whose contraction moves a part of the body directly.
Antagonist is a muscle whose action counteracts that of agonist.
Biochemistry
Agonist is a substance which initiates a physiological response when combined with a receptor.
Antagonist is a substance which interferes with or inhibits the physiological action of another.
Drug
Agonist is a drug which imitates the action of neurotransmitters in the brain.
Antagonist is a drug that blocks neurotransmitters.
Literature
Agonist is the leading character in a literary work.
Antagonist is the main opposing force of the protagonist.
Image Courtesy:
“Oliver Twist” by George Cruikshank – Oliver Twist by Charles Dickens via Folio Society (Public Domain) via Commons Wikimedia
“Villain” by J.J. at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
