toxin vs poisonous substance

toxin and poisonous substance both are nouns.

Word	Noun	Adjective	Verb	Adverb
toxin	Yes	No	No	No
poisonous substance	Yes	No	No	No

As nouns, poisonous substance is a hypernym of toxin; that is, poisonous substance is a word with a broader meaning than toxin:

toxin: a poisonous substance produced during the metabolism and growth of certain microorganisms and some higher plant and animal species
poisonous substance: any substance that causes injury or illness or death of a living organism

Other hypernyms of toxin include poison, toxicant.

toxin (noun)	poisonous substance (noun)
a poisonous substance produced during the metabolism and growth of certain microorganisms and some higher plant and animal species	any substance that causes injury or illness or death of a living organism