"It's tough to stay sharp, so I take 5-Hour Energy."

Energy drinks. Everyone from Tim Tebow …

"They said I couldn't win a national championship."

to 50 Cent and Joan Rivers …

"Hit me, Fiddy."

... are advertising them. With their Facebook pages and Internet video campaigns, manufacturers specifically target young people. But Consumer Reports says you have to be careful how much caffeine you drink.

Gayle Williams with Consumer Reports explains, "It can quicken your pulse, cause abnormal heart rhythms, keep you from sleeping well, and elevate your blood pressure."

Consumer Reports analyzed the caffeine content of 27 top-selling energy drinks - testing three samples each. Although some list the amount of caffeine on the package, they're not required to. Consumer Reports found the numbers can be way off.

Gayle says, "Some of the energy drinks underestimated the amount of caffeine listed on the label by 20 percent or more."

So how much caffeine do energy drinks contain? In Consumer Reports' tests, it varied widely.

For example, F-R-S Healthy Energy averaged 17 milligrams per container. Red Bull and S-K Street Kings Energy around 80.

Five-Hour Energy - 215 milligrams and 5-Hour Energy Extra Strength - 242.

Most healthy adults can consume up to 400 milligrams of caffeine a day.

Gayle says, "So for many people, an occasional energy drink is probably okay."

Or you can drink regular coffee. An eight-ounce cup contains roughly 100 milligrams of caffeine.

Consumer and scientific groups have urged the Food and Drug Administration to require companies to disclose caffeine levels, but the agency says it lacks the authority to do so.

Many energy drinks do carry warnings that they are not for children, women who are pregnant or nursing women, or people sensitive to caffeine.