Methamphetamine

Methamphetamine (also referred to as Speed, Crystal and Meth) is a drug featured in Grand Theft Auto V. It is mentioned several times in Grand Theft Auto IV and its episodes.

Grand Theft Auto IV and Episodes
Methamphetamine is briefly brought up in a conversation between Niko and Playboy X in the mission Blow Your Cover. Playboy X says that the two biker gangs in the city, The Lost MC and Angels of Death, are in the meth business. Later in the game, Ashley Butler visits Ray Boccino as he is talking to Niko, and she is evidently a meth addict, much to Ray's vexation. It is later revealed in The Lost and Damned that Ashley is Johnny Klebitz' ex-girlfriend. He dumped her because of her addiction and she is repeatedly used by Ray in order to get his way with Johnny and The Lost.

Grand Theft Auto V
Methamphetamine plays a much larger role in Grand Theft Auto V. Trevor Philips is a frequent user of the drug, making him highly strung and prone to violent outbursts. He also sells the drug in the Sandy Shores area, possibly explaining its economic decline and desolation. When Johnny Klebitz, who moved from Liberty City to San Andreas with The Lost and became addicted to meth, is brutally killed by Trevor, Trevor proceeds to kill the rest of the gang along with the Aztecas, both of whom were in the meth business, to make his self-proclaimed "Trevor Philips Industries" the only pusher of the drug in Blaine County.

When the O'Neil Brothers, also meth pushers and associates of Trevor, steal his business by winning over a Triad drug cartel wishing to partner with an outside organisation, he goes into a rage and destroys their meth lab. They later attempt to exact revenge but are killed in a joint effort by Michael and Franklin for Trevor.

Outside of the story, mobile meth labs are often seen parked throughout the countryside in GTA V. After the Mrs. Philips stranger mission, Trevor will be sat in his trailer smoking meth. He will then be in a drunken state for several minutes while playing, with the screen being cloudy and distorted.