I've been using crystal meth for about 12yrs what are the most common illnesses?

Meth abuse/addiction: If using meth for 12 years, you could probably educate us about the results you've had. Meth destroys tissues, blood vessels, and minds. Increased anger, mood instability, & psychosis. Acne, teeth grinding, decay and rotting teeth (meth mouth) appear. If used iv, exposure to hepatitis b & c, & hiv. Liver damage, seizures, stroke, extreme increases in temperature --> brain damage. Just a start.

Plenty: The biggest problem is mental health. Meth users are frequently misdiagnosed with paranoid disorder or bipolar disorder when they aren't truthful with the doctor. Dental problems are also significant. Sometimes liver and skin problems as well. The real challenge for you is to find someone to help you on that tough road to abstinence. It will likely require medical management + support groups.