This is my first day on the atkins diet and I need to know if I drink a diet dr. pepper a day, will it affect the diet?

It may or may not effect you.

Many of us are sensitive to artifical sweetners and caffeine and they slow or stall weight loss. In fact...Atkins says:

"Caffeine mimics the effect of sugar on blood glucose levels by stimulating insulin release. It should be avoided by those who suspect they are caffeine dependent and taken in limited quantities by others."

and on A/S...

"I have taken scores of aspertame users who seemed to be metabolically resistant to weight loss off this sweetner and observed weight loss to resume."

Hope this helps you make a decision.

Marlaine