Consumer Reports investigates caffeine levels in energy drinks

(NBC News) - Consumer Reports is out with a new investigation on how much caffeine they put in energy drinks.. And why you may not be able to believe the labels.

The headlines this week have been full of warnings about the caffeine levels in those high energy drinks.

Now, Consumer Reports has tested 27 products and found that:

- Five of the cans that list caffeine, actually had caffeine levels at least 20% higher than what their labels indicated.

- Eleven of the products don't specify their caffeine levels at all. Since they're considered a dietary supplement, they're not required to.

"It's important for people to understand what they're buying and drinking," says Gayle Williams, CR Deputy Health Editor, "and if that information is not even available on the product, that's a problem."

Among those drinks that don't list caffeine levels - "Monster." 

Last March, 14-year-old Anais Fournier died of cardiac arrest after drinking two Monster drinks - roughly 480 milligrams of caffeine, five times what's recommended for teens - in less than 24 hours.

The official cause of death:  "cardiac arrhythmia due to caffeine toxicity."

While she did have an underlying heart defect, her mother is now suing Monster.

"She's the last thing I think about at night. I visit the cemetery every day...," says Wendy Crossland, Anais' Mother.

Monster insists its products are safe, telling NBC News it "is unaware of any fatality anywhere that has been caused by its products."

But the FDA this week confirmed Anais is one of five deaths that could be linked to energy drinks.

Monster points out that its biggest drink contains LESS caffeine than many coffee drinks.  But as high energy drinks have exploded in popularity, ERs across the country have reported an explosion in caffeine overdoses."

"It's about body weight!  You know a small person and a certain amount of that drug, caffeine, and those stimulants may have a much different effect in a large person, an adult person perhaps," says Dr. Allen Taylor, Georgetown Univ. Hospital Chief of Cardiology.

With kids in mind, congress is now talking about requiring these drink producers to disclose their real caffeine levels.