Neither Diet Coke nor any of its ingredients are considered harmful to most people. However, Diet Coke and other sodas are correlated with a number of unhealthy conditions such as obesity and high blood pressure. This correlation does not imply that Diet Coke is the thing causing these conditions, however.
Most of the speculation about whether Diet Coke is harmful or not comes from controversy surrounding the artificial sweeteners the product uses. One of these sweeteners, aspartame, is one of the most extensively studied food additives on the planet. The U.S. Food and Drug Administration maintains that aspartame and many other artificial sweeteners are safe for use in food products.
Diet Coke has also gained attention as it may be related to patterns of overeating. There are many different hypotheses as to why this relationship exists, but a few are more compelling than others. One hypothesis suggests that artificial sweeteners cause the body to have an expectation of calories because of their sweet taste. When these calories do not come, this hypothesis suggests that people become hungry and start eating.
A different hypothesis suggests that Diet Coke and other diet sodas contribute to weight gain in a psychological way. By causing people to believe they are making a healthy choice by drinking a calorie-free soda, these beverages may lead people to make poorer choices regarding snack foods and overeating.