Drinking a reasonable amount of diet soda a day, such as a can or two, isn't likely to hurt you. The artificial sweeteners and other chemicals currently used in diet soda are safe for most people, and there's no credible evidence that these ingredients cause cancer.
Some types of diet soda are even fortified with vitamins and minerals. But diet soda isn't a health drink or a silver bullet for weight loss.
Although switching from regular soda to diet soda may save you calories, it's not yet clear if it's effective for preventing obesity and related health problems in the long term.
Healthier low-calorie choices abound, including water, skim milk, and unsweetened tea or coffee.