Each poison has a potency that determines its general effectiveness. Potency is usually based on some value of the poisioneer, such as the Medicine, Sorcery or Creature Powers skill. Poisons are generally resited using Constitution.
Poisons can be either slow or fast, and have a wide variety of effects.
Fast Poison
When you damage somebody with a poisoned attack, the poison will inflict additional damage at the end of the sequence. A victim only takes damage from the worst poison at the end of each sequence. The base damage is the poison's potency, and the victim soaks using Constitution. If prepared and using an antidote, he can substitute his doctor's Medicine skill.
Slow Poison
Out of combat, the main problem is delivering the poison. Any witnesses (including the victim) can make Medicine or Investigation rolls to recognize the poison for what it is, but only when somwhow subjected to it - tasting the poisoned food, coming close to the contact-poison door handle etc. When successfully poisoning someone with a non combat poison (slow, deadly poisons), your victim must then make a Death Test using Constitution or Medicine skill with a difficulty equal to your poison potency skill. After half the time till death has elapsed, your victim becomes violently ill, but before that, there are no obvious effects.
Antitoxins
Until the poison takes full effect (at the end of the sequence for a fast poison), it can be neutralized by quick medical action. Make a healing roll (usually with Medicine or Sorcery) against the potency of the poison; this roll can substitute for the patients Constitution roll when resisting the poison.
If you know you are going to encouter a specific poison, antitoxins can be taken in advance, allowing you to substitute you Constitution with Medicine Skill.
Poison Effects
There are many types of poisons. When somebody fails a death test caused by poison, they usually do not die. A paralyzing poison would cause either total rigidity or muscle collapse. Poisons can also cause sleep, disease-like symptoms or coma. Subtle poison require a Medicine or Investigation check vs. your Medicine action value to realize what happened. A hallucinogen causes the victim impairments, delusions and uncontrolled behavior. Many diseases, and the Blast effect disease, work the same way. Instead of inflicting damage, these inflict an accumulating Disability Rating. For every full five points of Disability Rating, the victim suffers one point of impairment. Any doctor can try to reduce Disability Rating, make a Medicine check against the Disability Rating, and reduce the Disability Rating by your outcome. This can be done once per day per disability.