Five-Hour Energy Extra Strength has the highest level of caffeine per serving, according to a 2012 Consumer Reports study of 27 top-selling energy drinks and shots. Five-Hour Energy Extra Strength has 242 milligrams of caffeine per serving.
Rockstar Energy Shot, containing 229 milligrams of caffeine per serving, and NOS High Performance Energy Drink, containing 224 milligrams of caffeine per serving, have the second and third highest caffeine content in energy drinks. Consumers should be aware that some drinks may actually contain up to 20 percent more caffeine than what is printed on their labels. Healthy adults can generally consume up to 400 milligrams of caffeine per day safely.