Caffeine is a mild stimulant that is added to soft drinks. When consumed, caffeine stimulates the central nervous system, which can make you feel more alert or even jumpy. According to the McKinley Health Center, 300 mg of caffeine per day is a moderate consumption level. However, caffeine affects each person differently—some may experience severe withdrawal effects, such as headache, fatigue or drowsiness. For this reason, it is important to be aware of how much caffeine you are consuming when you drink soft drinks.
The U.S. Food & Drug Administration regulates how much caffeine a 12-oz. drink can contain, according to the Center for Science in the Public Interest, which is 71 mg per serving. Caffeinated soft drinks that contain this limit or nearly this limit include Vault, Jolt Cola, Mountain Dew MDX and Coke Blak. This amount is comparable to one shot of espresso, which contains 75 mg of caffeine. A 16-oz. coffee contains 320 mg of caffeine.
Soft drinks that contain between 40 and 60 mg of caffeine per 12-oz. serving include Diet Mountain Dew Code Red and Diet Mountain Dew, both of which contain 54 mg of caffeine per serving, according to Good Housekeeping, and Mello Yello, which contains 53 mg. Many caffeinated beverages contain between 40 and 50 mg of caffeine, including Diet Coke, TaB, Diet Dr. Pepper and Dr. Pepper.
Soft drinks containing less than 40 mg of caffeine per 12-oz. serving include Coca-Cola and Coca-Cola Zero, which both contain 35 mg of caffeine, according to the Mayo Clinic. Diet Pepsi and Pepsi contain between 36 and 38 mg of caffeine per serving. Pepsi Wild Cherry contains 38 mg. Barq's Root Beer contains about 22 mg of caffeine per 12-oz. serving.
Some soft drinks do not contain caffeine. These include 7-Up, Fanta, Fresca, Sierra Mist, Mug Root Beer and Sprite. All flavors and versions of these contain no caffeine.