(I remember a quest where Geralt feeds a normal human a small dose of one of his potions.

witcher.fandom.com/wiki/On_Death's_Bed

Sadly, none of this denoted a happy ending. Though the wounds on Lena’s body healed, the toxins in the witcher’s brew had melted her mind. This was not the first time a cure had proven worse than the disease

The witcher potions are RIDICULOUSLY toxic