Yes, you CAN say "poisonous snakes"

You have probably been told that it is incorrect to refer to "poisonous snakes" (or spiders, etc.) because "poisonous" means containing poison, and thus poisonous to eat. They must be called "venomous snakes," meaning able to inject poison (venom).

Actually, however, in English "poisonous" has been used in both senses—poisonous toadstools, poisonous snakes—for a very long time. It's true that "venomous" has (at least in modern usage) been less ambiguous, but "poisonous" has always been common usage. The Oxford English Dictionary gives as the primary definition of "poisonous" "Containing, or of the nature of, poison; having the properties of a poison; venomous." That is, "poisonous" can simply be a synonym of "venomous". This use dates from at least 1563 ("poysenous beaste") and appears in Shakespeare.[1] The Texas Parks and Wildlife Department, who ought to know a bit about the subject, published a bulletin Poisonous Snakes of Texas.[2]

So if you say "poisonous snakes" you are in good company. Poisonous/venomous is indeed a logical distinction, but in normal use context is amply sufficient. If you see a snake and someone asks if it is poisonous, you can be fairly confident that they are worried about being bitten, rather than about whether to cook it for lunch.

Language follows it own eccentric logic, which is a never-ending source of annoyance to some people. A few oddities or ambiguities like this one attract attention, for no obvious reason, while many others pass unnoticed. Scientists need precise language, but it is a mistake to think that everyone should change their language to match the technical terms of science.