The energy drink you consume may contain more caffeine than you know, according to new testing by Consumer Reports.

Health concerns about energy drinks were heightened last week with reports of five deaths possibly linked to Monster Energy drinks.

The family of one victim, a 14-year-old girl, is suing the company that produces Monster, claiming she died after drinking two 24 oz cans, each containing 240 mg of caffeine. This is the equivalent of four or more cups of coffee or bottles of caffeinated soda.

The company issued a statement that it did not believe its product was in any way responsible for the death. Before the FDA story broke, Consumer Reports asked Monster why it didn't list caffeine levels. A representative replied, "because there is no legal or commercial business requirement to do so, and because our products are completely safe, and the actual numbers are not meaningful to most consumers."

Caffeine is a key ingredient in nearly every energy drink on the market, but it can be hard to tell how much they contain. Consumer Reports has just conducted lab tests on dozens of top-selling drinks.

Everyone from Tim Tebow from 50 Cent and even Joan Rivers 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.

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

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.

"Some of the energy drinks underestimated the amount of caffeine listed on the label by 20 per cent or more," Williams said.

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

For example, FRS Healthy Energy averaged 17 milligrams per container. Red Bull and Street Kings Energy around 80; 5-Hour Energy had 215 milligrams and 5-Hour Energy Extra Strength had 242.

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

"So for many people, an occasional energy drink is probably OK," Williams said.

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.

Other research suggests that more than 13,000 people a year visit U.S. emergency rooms because of symptoms associated with these drinks, and thousands more call regional poison control centers.

Health Canada used to classify energy drinks as natural health products because certain brands made health claims abut how they could restore alertness.  

That changed last fall, when the agency announced new regulations which will force manufacturers to say what’s in those energy drinks, including the very high caffeine content.  

Health Canada has given the energy drink makers a transitional period to get up to speed. Labels should be present on all drinks by late 2013.