Toxicity, venomous and poison
Solution 1:
You ask whether venomous and poisonous are (or are not) interchangeable.
As you and the many people who have posted comments point out, the two words are being distinguished scientifically: venom is injected into the bloodstream; poison is ingested, inhaled, or absorbed.
However, there are centuries of use, including modern use, where the two words are used, if not interchangeably, to mean the same thing.
Consider the OED's first definition of venom as a noun:
a. The poisonous fluid normally secreted by certain snakes and other animals and used by them in attacking other living creatures.The venom of snakes is secreted in a poison gland communicating with the fangs, through which it is ejected in the act of striking.
The OED's third definition of venomous as an adjective is:
a. Of animals, esp. snakes, or their parts: Secreting venom; having the power or property of communicating venom by means of bites or stings; inflicting or capable of inflicting poisonous wounds in this way.Formerly in general literary use, now chiefly restricted to certain species of poisonous snakes.
The OED's first definition of poisonous is:
- Containing, or of the nature of, poison; having the properties of a poison; venomous.
So the OED itself is using each word to, in part, define the other. Both have, as the OED points out in many definitions and quotations, figurative uses.
In a biology class, the professor would clearly take points off for calling a venomous snake "poisonous," but in ordinary English, this use is acceptable and standard.