Boomshine is an extremely volatile illegal alcoholic drink featured in the Grand Theft Auto series. It featured in Grand Theft Auto: Vice City, Grand Theft Auto: Vice City Stories, and on Grand Theft Auto III's Liberty Tree article. Boomshine's fumes alone are apaprently enough to make a person drunk. It is a take on the alcohol "Moonshine", which is also illegal.
In GTA Vice City Stories, Phil Cassidy had a cache of Boomshine in a storehouse in Little Havana. Phil asked Vic Vance to drive him because of a threat of an attack by the Cholos. Upon arrival, Phil noticed the door was open. He lifted up the door and was blown back by an explosion. Phil, knowing the volatility of the liquid, knew that should the Boomshine get heated up, a massive explosion would follow. Vic decided to save the liquor using a Forklift to avoid a large explosion and the loss of Phil's Boomshine. This cache would later be used by Phil to defend himself and Vic from Jerry Martinez's goons.
In GTA Vice City, Phil would be at his property in Little Haiti (most likely given to him by Vic, as it is owned by him in 1984) testing an explosive he wanted to show Tommy Vercetti. Phil, in his drunken state, did not take any caution in preventing a possible accident. The result: Phil got his arm blown off. The Boomshine's ambient fumes had given Tommy a drunken haze but he still decided to drive Phil to a hospital. On their way, Phil recollected some war memories during the drive (possibly giving him the idea that he lost his arm in Vietnam in Grand Theft Auto III).
Also, in 1986, a crazed fan planned to blow up the Love Fist tour limo with boomshine (which originally appeared to be Vodka) that had wires sticking out of it.
In GTA III, people continue to produce Boomshine in illegal distilleries around the city. The drinks were perceived to have driven the homeless community insane.