mannitol vs water pill

mannitol and water pill both are nouns.

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

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

mannitol: a diuretic (trade name Osmitrol) used to promote the excretion of urine
water pill: any substance that tends to increase the flow of urine, which causes the body to get rid of excess water

Other hypernyms of mannitol include diuretic, diuretic drug.

mannitol (noun)	water pill (noun)
a diuretic (trade name Osmitrol) used to promote the excretion of urine	any substance that tends to increase the flow of urine, which causes the body to get rid of excess water