For effects, we tend to look at what real-world poisons do to the people who get hit with them.
And anything outta nature's tool box is just... *shudder*
I did see someone's character once take about six hits of Adder venom; he nailed in the surprise round three times, one a crit that pretty much emptied the poison reservoir, then nailed a couple times again before the character could do anything. His legs essentially rotted off. There was nothing the healer could do for him but make him comfortable. There was simply no way for him to make the required saves, even with help - and you don't get poison-curing spells until spell level three or so.
It was a really fine lesson in 'don't get hit with that' - and I'd been watching a lot of 'idiots who keep venomous snakes' (paraphrased - can't remember the actual name of the show) before that. You can imagine the mental images /I/ had from that...