side effect vs fallout

side effect and fallout both are nouns.

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

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

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