soporific vs somniferous

soporific is a noun but somniferous is not a noun.

Word	Noun	Adjective	Verb	Adverb
soporific	Yes	Yes	No	No
somniferous	No	Yes	No	No

As adjectives, soporific and somniferous are synonyms defined as:

Other synonyms of soporific include hypnagogic, hypnogogic, somnific, soporiferous.

soporific (noun)	somniferous (noun)
a drug that induces sleep

soporific (adjective)	somniferous (adjective)
sleep inducing	sleep inducing
inducing mental lethargy