protagonist vs agonist

protagonist and agonist both are nouns.

Word	Noun	Adjective	Verb	Adverb
protagonist	Yes	No	No	No
agonist	Yes	No	No	No

As nouns, protagonist and agonist are synonyms defined as:

protagonist (noun)	agonist (noun)
a person who backs a politician or a team etc.	(biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction
the principal character in a work of fiction	a muscle that contracts while another relaxes
	someone involved in a contest or battle (as in an agon)
	the principal character in a work of fiction