The long-term effects of diet soda, although not widely studied, can include such things as cancer and diabetes. Aspartame has been shown to cause cancer in high doses--if the substance builds up in the body over time, what will happen? Sucralose, commonly known as Splenda, is just as bad as sugar when it comes to causing diabetes in patients over time.
Some of the chemicals found in diet soda are actually banned in some places because they have been found to be harmful on the body and linked to severe health problems like cancer. They are also known to help increase weight gain in studies, as dieters often eat more calories when they believe they haven't had as many with their diet sodas.