Dragons have been featured relatively frequently throughout the Harry Potter series. We know they are dangerous creatures due to their size, ferocity, and fire-breathing abilities. Is there any canon evidence that dragons are poisonous in any way, for example their bite, hide, claws, or internal organs?
Yes, although not so venomous as to be lethal (at least quickly). Ron is bitten by Hagrid's dragon in the first book:
By the next morning, Ron's bitten hand had swollen to twice its usual size. He didn't know whether it was safe to go to Madam Pomfrey -- would she recognize a dragon bite? By the afternoon, though, he had no choice. The cut had turned a nasty shade of green. It looked as if Norbert's fangs were poisonous.
As far as I recall, the dragons we see in the main books are not venomous, like the basilisk is, nor are we informed that they are poisonous.
Venomous, of course, refers to creatures which inject venom (like cottonmouth snakes or the basilisk). Poisonous refers to creatures which excrete poison that is harmful to touch or ingestion (like poison dart frogs).
The closest we see is when Hagrid has been beaten by
his full-giant brother
and has a black eye. He places a dragon steak on it. Hermione comments on how the meat doesn't look good to eat. Hagrid's comment is along the lines of, "That's ok, I don't plan to eat it."
Thus, there's no evidence in canon that they ARE poisonous, but there's circumstantial evidence that their meat is not healthy to eat (though one must wonder where he GOT the dragon steak - Hagrid is unlikely to have killed a dragon, and it would be rare for a store to sell food that's known to be poisonous).
As Kevin pointed out in another answer, Norbert(a), a Norwegian Ridgeback dragon was venomous - Ron suffered a bite from the dragon when it was very young, and it caused his hand to swell and turn green.
The Peruvian Vipertooth - a close relative of the Ridgeback - is also venomous.
Based on how a quick google search will show you the definition of poisonous then yes they are by a guaranteed 2/3 and probably all 3..
Google says Poisonous is...
(of a substance or plant) causing or capable of causing death or illness if taken into the body.
(of an animal) producing poison as a means of attacking enemies or prey; venomous.
extremely unpleasant or malicious.
Dragon - Peruvian Vipertooth
A small, very dangerous dragon species native to Peru.
habitat: mountains (?), probably eastern or northeastern Peru (see notes below)
appearance: smooth, copper-colored, short horns
size: small (15 feet)
food: goats, cows, but especially humans
Site References from Canon