WASHINGTON, DC (WUSA) -- The Food and Drug Administration is investigating five deaths that have been linked to energy drinks over the past three years.

The probe comes after 14 year old Anais Fournier died of cardiac arrest after drinking two 24-ounce cans of the Monster Energy drink.

Fournier's parents claim that the beverage company failed to warn consumers about the drink's dangers.

"The family hopes they can keep this from happening to anybody else," says attorney Kevin Goldberg.

Monster Beverage says it is saddened by the teen's death, but does not believe its drink is responsible.

A FDA spokesperson says that while the agency is looking into the incidents, reports do not necessarily prove that the drinks caused the deaths. But it does take them very seriously and will diligently investigate.

Caffeine is the key ingredient in the Monster Energy drink. And, according to the lawsuit, Fournier consumed a combined 480 milligrams of the stimulant.

Experts say that's equal to about 21 eight-ounce servings of Coca Cola or 19 eight-ounce Pepsi drinks.

But, we've found that it can be hard to tell how much caffeine these energy drinks contain.

Everybody from Tim Tebow to 50 Cent and Joan Rivers advertise them.

Manufacturer's Facebook pages and Internet video campaigns specifically target young people.

But, you have to be careful how much caffeine you drink.

"It can quicken your pulse, cause abnormal heart rhythms, keep you form sleeping well, and elevate your blood pressure," says Gayle Williams.

Consumer Reportsanalyzed the caffeine content in 27-top selling energy drinks, testing three samples each.

Although some list the amount of caffeine on the package, they're not required to. And, the numbers can be way off.

Williams 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?

Tests results show it varies widely. FRS Healthy Energy drink averaged 17 milligrams per container.

Red Bull and SK Street Kings Energy around 80. Five Hour Energy, 215 and Five Hour Energy Extra Strength, 242.

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

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

Or drink an eight ounce cup of regular coffee. It has 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, or people sensitive to caffeine.

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