“Poisonous—often confused with venomous—means a plant, animal, or substance capable of causing death or illness if taken into the body. Venomous means capable of injecting venom. A rattlesnake is not itself poisonous, because if you eat one it won’t poison you. A blowfish will kill you if you eat it, so it is poisonous, but not venomous.”
This is number six in Laura Hale Brockway’s list of “8 words that may not mean what you think they mean” on PR Daily. And it’s true that poisonous may not mean what you think it means, but this also implies that it may mean exactly what you think it means, and as it turns out, it does.
Though this was the first time I heard this complaint, it turns out to be mildly common. Paul Brians mentions it in his common errors — in fact, Brockway seems to have lifted half of her complaint from his. You can find a number of other online objectors, of course, but it’s uncommon in printed usage guides; of the seven within my reach at the moment, only Ambrose Bierce’s Write It Right complains about poisonous.
Conveniently, my edition of Write it Right is Jan Freeman’s excellent centennial edition, which means that each of Bierce’s complaints is accompanied by her research into it. About this issue, she writes:
“As usual, Bierce would like to fence the overlapping words into separate pens. But while venomous does describe rattlesnakes and other animals that poison victims with a bite or sting, poisonous has always been a broader term. Samuel Johnson knew both words, but in his Dictionary (1755) he referred to ‘a poisonous serpent,’ ‘a poisonous insect,’ and ‘a poisonous reptile.’”
It’s not just Johnson, either. The Oxford English Dictionary cites The Indian Queen, a play by Robert Howard and John Dryden (he of “no final prepositions” fame), with “poisonous Vipers” in 1665. Google Books can supply you a vast array of hits for “poisonous snakes” from the 1800s, if you need convincing of the lineage. Here’s my favorite, as it’s very clearly talking about snakes with venomous bites; it’s written by someone studying the venom of the snakes, so this isn’t some casual imprecise usage but the considered usage of a professional; and it’s from 1839, so there’s no arguing that this is some sloppy modern usage.
In short, the two words do not have distinct meanings; rather, one has a subset of the other’s. This is common in English; I’ve previously written about jealousy/envy, verbal/oral, and compose/comprise, all of which display this to some degree.
In the case of venomous and poisonous, this oughtn’t to be surprising, as their stems have this same relationship. A venom is one kind of poison, and similarly, being venomous is one way that an animal can be poisonous. The biggest clue that we aren’t all wrong for using poisonous in place of venomous is that it’s very rare to see the opposite extension. When people talk about “venomous plants”, for instance, they’re usually talking about plants that literally do sting, like stinging nettles or the gympie gympie. If people are just stupid or underinformed, they ought to make their errors symmetrically; here, the supposed error really only goes one way. (I’d expect asymmetric errors if one were much rarer than the other, but venomous isn’t particularly rare.)
So poisonous and venomous overlap in general usage, and I’m having trouble seeing why anyone would expect or even want them to be separated. The only situation where it would potentially be worth having distinct definitions is if you’re regularly dealing with things that contain poisons delivered by different methods. But if that’s the goal, poisonous and venomous don’t supply enough categories. Having poisonous describing anything but venomous is just strange, given that it doesn’t make presumably critical distinctions between the poisoning methods of, say, tree frogs (touch) and pufferfish (ingestion).
Summary: Poisonous and venomous aren’t totally distinct. It’s fine to call a snake poisonous, even if it’s its venom that’s dangerous. But it’s rare (and generally incorrect) to call something with a non-venom poison venomous. This is how it has been for hundreds of years in English. Objections to the subset relationship between poisonous and venomous are pretty rare, and outside of specialized contexts, pretty unfounded.