The only problem towards dispension is that it must be taken daily in the set amount, for it to occur gradually over the week. Though, if not needed to occur almost naturally, it can be taken in lump sum to form the result overnight.
It's effects when taken gradually, will start with red blotches across the cheeks and nose. After a second day of the dose being in the system, the blotches become large pustuils, which will threaten to pop, but cannot do to their nature. The third day will result in the addition of large flakes of skin peeling off from the area on the forehead. The fourth day will result in the pustuils popping and oozing continuously through the day. The fifth day results in a particularly annoying and itchy rash forming on the chin. The sixth day the still oozing pustuils will begin to solidify and form hard cones, across the area they are at. The seventh day the entire face will swell twice it's size.
This potion would be taken in a single drop daily, mixed into food or beverage. It is unable to be distinguished from water by smell or taste or sight, unless mixed with human sweat, in which it will acquire a blue color, and the smell of rotten eggs. As stated before, it can be taken at once to give overnight results, or spread out as an effective poison. It is not however permanent, and once it is ceased to be ingested, in three days, all symptoms will be gone.
The ingredients for this potion are common, except for two. I have one of those two, and the second one I am aware of where to find. Including the time to find the second rare ingredient, and brewing time, it will take five days to complete. Instructions for brewing are complicated, and therefore I will create it myself, for you.
As compensation, I ask that you hold yourself debted to me, for a favor. When I have need of your services in the way of information, or spreading of news, I will call upon you.
If this deal is acceptable, owl me back, and I will begin the work to create the potion. When it is done, I shall track you down to deliver the flask of it.