Well it wasn't Mushrooms that fueled Berserkers

https://www.youtube.com/watch?v=vWKnK1HCuoU

It was Black Henbane which is cool and all but aparently the residual effects make you a PITA to live with and the Townsfolk may have to take you out.

Aparently administered before Battle it made you Howl, Foam at the Mouth and want to fight Uncontrolablly.

So, it was Jager Meister?