Why do people on meth lose their teeth?

Many reasons: "meth mouth" is caused by a combination of grinding teeth, dry mouth, bad oral hygiene, and increased sugared soda intake, all of which are side effects of methamphetamine addiction. Meth, like all "uppers" makes the body edgy and tight. The cost of prison dental care has exploded since meth became common. http://www.pbs.org/wgbh/pages/frontline/meth/body/.

Acid, Suger, Neglect: Meth is caustic to all tissues, including the teeth, which can't repair themselves after damage like the rest of the body. Meth users usually crave and consume lots of simple sugars which promote decay. They also neglect their entire body including their mouth. Put those three factors together and you have a perfect storm to destroy the teeth.