The drink is often in a glass bottle, but during certain cutscenes The Character occasionally drinks from a glass. The beverage seems to be alcoholic, since the screen gets hazy when the beverage is drinken. The name of the drink is wordplay on Poltergeist and Budweiser. The bottle looks like a 40 oz bottle of beer.
The beverage is used to make you walk though walls. This effect is temporary, and a certain future Gem renders the Polterweiser effect useless. This effect can help you perform suprise attacks on enemies.