fallout vs side effect

fallout and side effect both are nouns.

Word	Noun	Adjective	Verb	Adverb
fallout	Yes	No	No	No
side effect	Yes	No	No	No

As nouns, fallout and side effect are synonyms defined as:

fallout (noun)	side effect (noun)
any adverse and unwanted secondary effect	any adverse and unwanted secondary effect
the radioactive particles that settle to the ground after a nuclear explosion	a secondary and usually adverse effect of a drug or therapy