There's a post on The Dungeon Fantastic about poisons, and whether player characters should be allowed to use them. Here are my thoughts on the matter.
The primary problem with players using poisons is the same problem we have with monster venom: most poisons shouldn't be of the instant death variety. Even the poison used by tribes that hunt with poisoned arrows generally takes a couple minutes to work. They use poisoned arrows so that they don't have to chase wounded prey for hours, waiting for it to bleed to death. Certainly, the poisoned weapons in legend and literature don't seem to cause instant death, just wounds that never heal. However, we GMs are often lazy and just use instant death for everything, in which case we ought to apply the same rule to players using poison.
But the problem with that is that it seems to make player tactics super-effective, thus the attempts to limit the use of poisons, which often go too far. For example, poisons shouldn't last forever, but tracking the number of hits made with an envenomed blade is a bit tedious, isn't it? Definitely, there should be a difference between a rattlesnake, which constantly produces new venom while alive, and a sword with poison on the edge; but rather than making separate rolls to see if the poison gets wiped off or loses its effectiveness, it would be a lot easier to just say that when a victim makes a saving throw against the poison, the success is because the poison has finally been used up. In addition, the poison should only last a week when exposed to air, so that if a typical adventure crew makes weekly expeditions, they have to re-envenom their weapons every expedition. Did the players say "we put poison on our swords" and pay for any poison used? If so, they have envenomed weapons this week; otherwise, they don't.
The other notable restriction usually suggested for poisoned weapons is the risk of self-inflicted wounds or "friendly fire". This should definitely be an issue, since people who work with poisons do occasionally kill themselves and tribes that use poisoned arrows are extra careful about the arrows (as if, you know, they've had some bad accidents.) But again, the usual response is a whole bunch of extra die rolls. I think most of the time, you could limit the danger to characters who have been cursed (any miss with an envenomed weapon will automatically hit any cursed character whose near the victim.) Also, of course, the restrictions on firing arrows into a mêlée should definitely be in effect; you should probably apply those rules to blind fighters or fighting in darkness as well.
There are also the social effects of being a known user of poisons, but these are very role-playing-dependent and shouldn't take the form of automatic reaction penalties or guards spotting a poisoned weapon from a hundred yards away. So in short, I'd definitely allow poisoned weapons, but it would definitely be an added weekly expense with a few minor risks, not a cakewalk or a reason for continual punishment.