For a stress inducing Poison Potion, I would simply apply shifts to either power or duration, where 1 shift equals 1 exchange or power. A 12 shift potion could then be a poison 6 attack for 6 exchanges (which would give you the most shifts of damage). If you want a faster potion, it is not going to last that long, and if you want to have a poison that stays longer, it is going to be weaker, so it should balance out rather fine.
You might even look at another way to distribute the shifts, maybe a poison does 1 shift of stress in the first exchange, then 2, then 3, and so forth, slowly building up it's deadliness. Though you should not exceed the total stress it would be able to inflict if it behaved linear. And it might be worth 2 shifts to change it's behaviour like that.
Poisons that apply an effect like "sleeping" would best be build as a maneuver with the duration increased by the standard time table.
Usually, potions are ingested, so I would have that as a baseline means of delivery. If you want to do it another way, I would probably require 2 shifts for it to be able to be delivered for example through a weapon or as a gas or any other way you might think of.
Remember though, that killing with a magical poison is still killing with magic, thus violating the first law.