Many games have classes that make extensive use of poisons. Poisons of course have been used in murder and poisonous gases have been used in war. But how realistic are the poisoned weapons so common in many rpgs? By that I mean: Is there some poison which can realistically (or at least plausiblely) be applied to a weapon in a way that will actually affect the outcome of a battle?
An answer showing how poisoned weapons have been used in history would certain answer this, but its not necessary. I am more concerned about verisimilitude and whether it is believable to have non-magical poisons applied to weapons in an effective way. Is it realistic to poison a weapon in the sense that it would actually work? And is it realistic to have a reason to do that even if it would work?
This question is obviously related: Creating Poison Cheaply - Potato Tea? but asks something different and is more focused on game balance if you allow real world ways of making poisons.