Has anyone seen any rules for a venom or toxin that causes paralysis rather than damage?

I could run it similar to the toxic talent so that if the character is damaged by something the paralytic venom instead of taking additional damage they get their dexterity reduced by D5 until the venom is cured or wears off. Successive bites add to the dexterity penalty until eventually the subject is unable to move.

Or perhaps the fatigue rules are better to use and each bite adds to the fatigue total until the character passes out.

Any ideas or is there a rule I haven't noticed that already takes care of this?

The Dark Heresy Gamemaster's kit has rules for poison that can be a Sedative (fail a Toughness test, fall unconscious) or Paralytic (take Strength damage based on degrees of failure, 0 = unconsciousness).

Those seem to be viable options. Was it just a standard 'challenging' toughness check?

It's definable by the GM, though they do include many sample poisons. There is a sedative poison that's taken at a -20 Toughness Test, and intended as a warning that the next time it'll be a real poison, and a paralytic that takes at a -30.

The "Paralytic" species quality in the Xenos Generator from the Koronus Bestiary does this, and could be imported onto custom creatures if you don't want to generate a full species randomly.

Fatigue is something massively overlooked in most games ive played in and so im slapping on my players when i feel appropriate - that -10 to ALL tests is quite a big deal.

Yes, what 40KRPG always needed was save or die attacks.

Take Carouse, drink your way out of the problem.

Take Carouse, drink your way out of the problem.

This makes me think of a drug that can only be absolved with high levels of intoxicants to counter the effects. Super deadly and always kill, with the only known survivor having been a infamous retired (and often plastered) Imperial Guard sergeant who went on a drinking binge to save his life.

And was promptly executed for blasphemous defacing of imperial property in the aftermath of his antics. =D

This makes me think of a drug that can only be absolved with high levels of intoxicants to counter the effects.

Methanol?