As this is a popular question please see the below.....
I would say no to diet drinks as many are not calorie free. Our Cambridge drink mixes are based on Malic Acid which allows you to remain in ketosis whilst drinking them on the diet, but only one teaspoon a day!
Diet coke also contains citric acid and research has shown that citric acid can prevent the body utilsing itís carbohydrate stores and cause fluid retention.
However, once established on sole source and in ketosis, you could maybe have no more than 1 glass of diet drink per day as a one off treat if absolutely needed.
I hope this helps.