inhalation vs breathing in

inhalation and breathing in both are nouns.

Word	Noun	Adjective	Verb	Adverb
inhalation	Yes	No	No	No
breathing in	Yes	No	No	No

As nouns, inhalation and breathing in are synonyms defined as:

inhalation and breathing in: the act of inhaling; the drawing in of air (or other gases) as in breathing

Other synonyms of inhalation include aspiration, inspiration, intake.

inhalation (noun)	breathing in (noun)
the act of inhaling; the drawing in of air (or other gases) as in breathing	the act of inhaling; the drawing in of air (or other gases) as in breathing
a medication to be taken by inhaling it