"Throughout his life, the magePetri 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."
Petri's Philter increases the intensity of all the witcher Signs.
This potion, created by a student of a famous mage of the first Conclave, is weaker than Stammelford's Philtre, but has no side effects. Petri's Philtre increases the intensity of witcher Signs – the impact of the Aard, the defenses of the Quen, the effectiveness of the Yrden, and so on.
The potion increases Toxicity very markedly when imbibed.
It is recommended for witchers who rely more heavily on magic than on their swords, and before fights against monsters vulnerable to Signs.