water pill vs drug

water pill and drug both are nouns.

Word	Noun	Adjective	Verb	Adverb
water pill	Yes	No	No	No
drug	Yes	No	Yes	No

As nouns, drug is a hypernym of water pill; that is, drug is a word with a broader meaning than water pill:

water pill: any substance that tends to increase the flow of urine, which causes the body to get rid of excess water
drug: a substance that is used as a medicine or narcotic

water pill (noun)	drug (noun)
any substance that tends to increase the flow of urine, which causes the body to get rid of excess water	a substance that is used as a medicine or narcotic

water pill (verb)	drug (verb)
	administer a drug to
	use recreational drugs