Houserule: Poison dosage

[Eldan]

It just occured to me that, with basic rules, the size of a creature doesn't matter when regarding the amount of poison needed to kill it. Especially, I thought about making a poison that would kill rats, but not humans, which is difficult, since both have about the same average constitution.

So, what about a houserule by which poisons affected creatures differently based on their size? Perhaps a bonus/malus on saving throws based on size category, or simply adjusting the amount of poison needed to kill certain creatures, so that humans would automatically pass the save against the amount of poison necessary to kill a rat?

[Harperfan7]

This is already somewhat covered. In the monster manual, when you increase a creature's size, you increase it's constitution and HD, and thereby fortitude saves.

[Random832]

The problem is it makes no sense for a rat (Fort save +2) to require more poison to kill than a 1st-level Commoner (Fort save +0).

[DracoDei]

Harperfan: Constition only increases when the final size is Large or larger I think. As for HD, there is a similar problem, because there is no difference between the base saves of a 1/20th HD housefly (or whatever you think 1/20 of a HD is) and a 1/2 HD creature. In fact, there might not be a difference between 1/2 and 1 HD

[Harperfan7]

Harperfan: Constition only increases when the final size is Large or larger I think. As for HD, there is a similar problem, because there is no difference between the base saves of a 1/20th HD housefly (or whatever you think 1/20 of a HD is) and a 1/2 HD creature. In fact, there might not be a difference between 1/2 and 1 HD

I know, I was responding to his post in general. That's why I said it was somewhat covered. It doesn't matter anyway if we're talking about poison on weapons, because rats only have 1 hp. (I think)

[paddyfool]

The problem is it makes no sense for a rat (Fort save +2) to require more poison to kill than a 1st-level Commoner (Fort save +0).

Depends on the poison. For instance, many rats have developed a resistence to warfarin following its widespread use, so a dose of that that would kill a human wouldn't necessarily bother a rat.

Overall, however, size really does matter. Real-life poisons generally have their lethal dose, or LD50, measured per kg mass of the target organism; the amount that is lethal is therefore directly proportional to the mass of the target organism.

[levi]

Overall, however, size really does matter. Real-life poisons generally have their lethal dose, or LD50, measured per kg mass of the target organism; the amount that is lethal is therefore directly proportional to the mass of the target organism.

LD50 is actually the dosage that kills half the test subjects.

[paddyfool]

Yes, it is. But that dose is usually measured in mg/kg, i.e. "How many mg of the poison to kg of the organism does it take to kill 50% of test subjects".

[AstralFire]

Every two size categories up halves the DC. Every two size categories down doubles it.