Meth Mouth...
I did a little poking around and found that the ingredients that go into making meth is what causes the tooth problems. I guess meth is made up of some pretty caustic stuff. I saw words thrown around like ammonia, red phosphorous, lithium, ether, and two type of acid - muratic and sulfuric!!!
Then Brianna mentioned dry mouth, this comes into play. No saliva to wash away the nasties. People on meth tend to crave surgary snacks for some reason - candy bars and soda. All this plus you're pbly not the most hygenic when you're trying to score your next hit will destoy you teeth. My brother and sister-in-law were on this stuff for a while and the effects on their teeth weren't this extreme but they have some major damge done. Sidenote: Bro and sis have been off this stuff for years now.