Common graytop brew

Common graytop brew is a very strong hallucinogenic made out of graytop (a nearly extinct mushroom) that Geralt can take if he wishes to help Pinastri during Equine Phantoms. When taken, it causes Geralt's vision to take on a mirage-like effect with a purple tint. The biggest benefit is that it allows Geralt to see Umbrae, which are invisible even to witchers under normal conditions.

There is no way to take the brew outside of the quest as Geralt will automatically take it if he says he'll help the hermit.