Not a synonym, at least by the nouns they're based on. Toxin is a parent category that encompasses both poisons and venoms.

Poisons are something you do to yourself (usually by ingestion), venoms are injected by the other organism (such as a snake bite).

I quoted from Wiktionary, feel free to update the definition there (with sources) if you disagree. Note that the "Synonym: toxic" appears underneath the second (figurative) definition, and in my opinion is perfectly correct; in figurative use poisonous and toxic are used in the same way.