Here is an excerpt from the Deadly Doses guide to poison strength, a couple of sample poisons, and a suggestion or two on how you might decide to incorporate all this into a level-free poison system for your game:
Deadly Doses Rating System (partial)
Category NumberCategory NameLethal QuantityLethal amount for a 150 lb human
6supertoxicless than 5mg/kgless than 7 drops
5extremely toxic5-50 mg/kg7 drops to 1 teaspoon
4very toxic50-500 mg/kg1 teaspoon to 1 ounce
Sample Poisons
Example 1: Celandine (also called felon wart, rock poppy, swallow wart).  The listing for this poisonous plant tells the reader that the entire plant is poisonous, particularly the leaves and stems. It is toxicity level 5. Symptoms take up to fourteen hours to appear and include: nausea and vomiting followed by coma and death. The plant must be swallowed to take effect.

Example 2: Cobra (and related snake) venom. This extensive listing begins with a physical description of the different venomous snakes in this family. The descriptions also tell of any special properties that the snake or its venom has. Following that, the authors inform us that cobra venom is a neurotoxin of toxicity 6. It kills by progressive paralysis that eventually stops the lungs and suffocates the victim, generally within two hours. Other symptoms include low blood pressure and possibly convulsions. The listing also includes descriptive case histories and the reader will discover that cobra venom is also lethal if swallowed or absorbed through the skin.

Level-Free Poisoning

        So how do you turn the Deadly Doses rating system into a poison system for RPGs? This is clearly easier to do in a level-free system like Hero. The character simply makes a CON roll at appropriate modifiers for the severity of the dose. Success means minimal lasting damage, failure means life threatening injury. But what about a level-based system like Rolemaster? For the purpose of this system (and, let's face it, realistically), the poison is always the same strength relative to the character's skill and experience level. So simply have the characters roll against a poison of their own level. For a first level PC, the poisons are all first level, for a tenth level character, all poisons are treated as though they were tenth level. And just like resisting a spell cast by someone of the same level, to succeed you need to break 50. So:
Percent of Lethal Dose1-10%20-40%40-60%60-80%80-100%
Suggested Rolemaster Con Roll Modifications+50+25+/- 0-25 
(optional: -50)
(optional: -75)
Suggested Hero System Con Roll Modifications-8-4+/- 0+4+8
Rolemaster: Character must roll and add Con bonus and any modifiers. Success results from breaking 50. 
For Hero, success means rolling under (CON/5)+9 after modifiers. 
Success means that, while the character may be incapacitated in the short term, he will suffer no lasting effects. Success 
by more than +25 can be taken to mean minimal acute effects as well.  I.e. the characters may feel rotten, but they can function effectively when it counts. 
For a more lethal system, simply increase the  detrimental modifiers. You could, for example, decide that a sequence of +50, +25, +/-0, -50 and -75 (optional suggested modifiers, above) is more suited to your world.  This has the advantage of only hurting super-stat characters.  Normal folk are going to fail at those doses regardless.

        So if our man Odo Strongbreath is careless while gathering veggies for the evening's salad and accidentally ends garnishing it with Celandine leaves rather than radicchio, he will need to make a roll against a poison of his own level at a lethal dose (-50). Odo does so, adds his Con bonus, and comes up with a total of 78. 78-50=28. Unless some more perceptive member of the group gives Odo an emetic for dessert, he will begin to feel nauseous the next day and soon collapse, near death. By then, without magical healing, he's a goner.  Better hope there's a healer near by, Odo....

Editor's Note

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