Methamphetamine (meth for short) is a psycho-stimulant that can be found occurring naturally in some variations of the acacia tree.
It is, however, most commonly produced by synthesizing certain caustic chemicals and OTC stimulants. The resulting product of this synthesizing process is a poisonous substance that includes not only the methamphetamine compound, but many additional toxins as well. In this form, it is much more commonly known as crystal, crystal meth, crack, etc.
Meth is very addictive as well as destructive. Withdrawal effects are similar to those of cocaine, amphetamines, and nicotine (to a much greater degree).
On Criminal Minds
Spree killer Eddie Mays (Blood Hungry), bank robber Caleb Dale Sheppard (Psychodrama), and murder victim Matthew Benton (Demonology) were all known addicts of meth. Serial killer and rapist Billy Flynn (Our Darkest Hour and The Longest Night) regularly smoked crystal meth in order to stay awake at night. Poisoner Larry Feretich (Brothers Hotchner) mixed meth with PMMA to create a lethal mixture, which he used to murder his victims through drug overdoses. All three members of the Outlaw Bikers (Outlaw) used meth to get high during their crimes.