If a snake’s head is cut off, and, assuming the venom sacs and fangs are not damaged, can it still be used to poison? I would assume that as an organic compound, the venom would quickly break down making it worthless after a while. How long of an interval do you have from the snake’s death to the time when its venom is neutralized? I’ve heard that if you preserve the head, you can use the poison indefinitely but I don’t know about the credibility of the source.
Well the answer is both yes and no. Until a certain amount of true putrefaction sets in a snakes venom is certainly still quite potent. I would go so far as to say that until the rot actually affects the venom glands, it is most likely still very dangerous. Dried venom can be just as potent as the fresh stuff once it is re-hydrated. There are plenty of stories of postmortem bites from idiots playing with the snake’s head.
There was a lengthy and informative thread about this a few months ago. I can’t figure out which one it was, though. There were lots of interesting anecdotes about people being poisoned by dead snakes because the venom remained in their fangs.
See the last few paragraphs of this article on Snopes.