Diet Dr. Pepper contains caffeine, which is unhealthy in high amounts. Consumption of four or more caffeinated drinks per day may cause side effects, such as stomach upset, muscle tremors and irregular heartbeat. Those with caffeine sensitivity may suffer from headaches and insomnia from even small amounts of caffeine.
Diet Dr. Pepper also contains aspartame, an artificial sweetener commonly used in soft drinks, prepared foods and medicines. There have been studies linking aspartame to certain cancers, but the U.S. Food and Drug Administration has not found these studies to be conclusive. Aspartame should, however, be avoided by patients with phenylketonuria, since it contains phenylalanine.