Ammo conservation. Also, perhaps noise minimization. A silenced .22LR pistol will be damn quiet, and a single poisoned bullet will do the job without costing a fortune.
Yeah i guess you could pretty much ignore my entire post...
I have been shot with a 22lr out of a rifle (a child at a shooting range, i could still hang his father), unless that poison can somehow instantly kill, that silencer was a waste of time... my leg still looks like someone put a cigar out in it. Also if it does work, be sure to notify your assailant not to harm you while your poison takes effect, it can really throw a wrench into your plans.
If you are going to go completely illogical and insist on this poisoned bullet nonsense, a subsonic round (which is what silencers are coupled with, fyi, huge difference between a silencer and a suppressor) which would reduce the heat in the barrel slightly. How this is all going to be absorbed into the bloodstream and how the bullet is going to be carrying a large enough quantity of it to be effective is still complete nonsense, but since you seem bound and determined, don't let it slow you down...