I retroactively added this post to the WCotFP series.
I am really not a fan of poison that instantly kills a character dead on a single failed saving throw, but I neither can say that I am very fond of the various mechanics from d20 games to deal with poison.
And completely out of the blue I suddenly had this idea for how one could possible handle poison in OSR games (and probably a wide range of others as well). It’s so simple that I am most likely not the first to come up with it, but that actually makes it a good argument for and not against it.
When a creature gets hit by a poisonous attack, it needs to make a saving throw against poison or take X amount of damage. At the begining of its turn, a poisoned creature has to make another saving throw or take another X points of damage. Once it successfully makes a saving throw against the poison, it takes no damag and the poison ends.
The strength of the poison is entirely defined by the amount of damage it deals. The difficulty of the saving throw is always the same (no penalty to the saving throw against very strong poisons) and the duration of the poison is always as long as it takes to make a successful safe. So you only need to remember the amount of damage done by the poison and nothing else. You don’t even have to take count of how long the poison has already been acting. Poisons that deal higher amount of damage are more difficult to survive simply by the fact that you might run out of hit points before you even get the opportunity to make a third or fourth attempt at shaking it off. Even if you survive, a high damage poison still leaves you a lot more crippled than one that deals little damage. And if you’re already injured and unable to take much more punishment, even a relatively weak poison might still kill you.
Since saving throws against poison in B/X are usually save or die, the chance to succeed are pretty good, even for 1st level wizards. The chance that you take damage three or four times before making the save are very low at any level and at high level getting damaged even twice won’t be very common. So because of that, the amount of damage dealt by the poison has to be pretty high. I think a good rule of thumb might be that the poison should deal at least as much damage as the primary bit or claw attacks of the creature. In case of a small creature that relies primarily on its poison, it should be even considerably higher than that. I wouldn’t even bother with anything under 1d6. The highest number I use with my monsters is 3d6 for wyverns, and that’s because I am always very generous towards players when it comes to poison. If you want really nasty ones, you could easily go up to 4d8 and beyond.