The hypothesized causes of meth mouth are a combination of MA side effects and lifestyle factors which may be present in users:

Dry mouth (xerostomia)

Clenching and grinding of the teeth (bruxism)

Infrequent oral hygiene

Frequent consumption of sugary, fizzy drinks

Caustic nature of methamphetamine (less likely: “Meth mouth is generally most severe in users who inject the drug, rather than those who smoke, ingest or inhale it.”)