I once met a client whose doctor had referred her to me for diet counseling. She drank 15 cans of Dr. Pepper a day and complained of some stomach issues. She threatened to sue Dr. Pepper. The remedy as you can probably tell: Both the doctor and myself told her to stop drinking the soda. Do you think she followed our advice? Probably not. Each can of Dr. Pepper has 64 grams of sugar. Diet Dr. Pepper has 123 grams of aspartame. For our 15 can/day situation, you can do the math and in either case, diet or not, there are concerns.
FYI: Here is the aspartame content, in order from least to most per 8-ounce bottle: Sprite Zero (50 mg), Coke Zero (58 mg), Pepsi Max (77 mg), Diet Pepsi and Caffeine-Free Diet Pepsi (111 mg and 118 mg, respectively), Diet Dr. Pepper (123 mg), Diet Coke and Caffeine-Free Coke (125 mg).
Whether the soda is diet or not, over indulgence can both have consequences. The following article from Fooducate discusses diet soda and offers a novel solution: WATER.