Aspartame (Nutrasweet) has a long list of side effects, as do most artificial sweetners! You can google Aspartame side effects, I believe you will find your answers there.
The best sweetner to use is Stevia, some brands have an after taste....I use one called Astraya Stevia, its sweet and tasty. Stevia has many favorable health benefits and it is completely non-toxic. The herb is actually nutritious, contains protein, calcium and many other vitamins and minerals.
If you need some help getting the water down and to quench your thirst try making Lemonade or Limeade from Fresh lemons or limes, sweetened with Stevia. Serves 2 purposes will quench your thirst and lemons/limes are very alkalizing to the body.
I have a friend who drank up to 6 cans of diet coke a day and had symptoms of a stroke, it turned out to be the Nutrasweet laden DC. She started on a detox diet of fresh fruit and vegetable juices for 10 days and then stuck with a fresh food diet and had a complete clearing of all symtoms. It was very scary to say the least.
Hope this helps.