My teeth have gone brown since drinking so much diet coke is it the coke or something else?

See a dentist: Hard to determine if it is just staining or cavities without being able to see your teeth.

Need hygiene visit: Go to your dentist and have a professional cleaning. If the stains come out, your good. The dentist will do a clinical examination in addition to x-rays if needed, to make sure no additional issues exist.

Diet Coke Habit: The damage from excessive Diet Coke consumption can cause deep brown stains. Prevention and/or moderation is the best cure. See your dentist for examination, hygiene and your dentist can offer educational tips.