Very many people who use meth get cavities and also often have untreated tooth decay. The term "meth mouth" refers to the stained, badly damaged teeth common among people who use it.
One reason for the damage is that the drug is acidic and causes vomiting and reflux. And even though it makes you less hungry, it makes you crave soda and sweets.
The damage can happen in a short amount of time and often leads to the loss of teeth.