Sherm has absolutely nothing to do with PCP.
Sherm is a
joint or simply a cigarette that has been dipped in embalming fluid (aka. formaldehyde) and left to dry. It must not be lit using an open flame lighter, but rather an automobile cigarette lighter is
perfect to avoid an unwanted flame-up in one's
face.
Use of sherm causes brain damage, and with prolonged use this permanent brain damage becomes increasingly serious, and the user then gets referred to as a shermhead.