Not all sugar-free carbonated drinks contain alternative sweeteners, so check the ingredient list for aspartame, saccharin, sucaralose and acesulfame potassium.
What they do have are possibly carcinogenic food colorings, tooth-enamel-eroding flavors, and additives, and the sweeteners like aspartame can create elevated risks of heart disease, stroke, and type-II diabetes.
In order to exceed the daily limit for aspartame, an adult weighing 60kg would have to consume somewhere between 12 and 36 cans of fizzy diet drink per day.