There is no potion and no toxicity level that takes half of your vitality away. Others have explained that. It is an artifact of using the Full Moon potion, which doubles your maximum vitality but does not increase your current vitality.
Various levels of toxicity cause the red spots to appear, cause you to miss more often, cause you to be knocked down when casting signs, cause you to be unable to cast signs, cause you to be unable to run. At toxicity over 75%, you start losing vitality over time. The game is designed to do this.
The usual strategy for playing the game as a potions addict is to use potions with dominant albedo. When you take a series of potions, start with one that has dominant albedo. The toxicity of that potion, and all of the others you take before the albedo wears off, is reduced.
You should also be prepared with a few White Honey potions and have put a few talents into your Quen sign. Then, if you get in over your head, cast Quen, drink a White Honey to detox, and drink a new round of potions.
If you want the game to perform differently, you will have to learn to use Djinni to modify your game. You should start with the Djinni tutorial; you can find it via http://djinni.wikia.com/wiki/Main_Page