Petri

From Witcher Wiki

Throughout his life, the mage Petri sought a way to strengthen his magical powers. He failed in this endeavor, but several interesting potions came about through his attempts. One of these, known as Petri's Philter, is useful to witchers with magical skills. Witchers who prefer hand-to-hand combat rarely use Petri's Philter, however, especially since it is highly toxic and thus renders the use of any other potions impossible.