North Americans spent over half a billion dollars on energy drinks last year.

We tested more than a dozen energy drinks, including Red Bull, Full Throttle and Rock Star, to see what is charging their popularity - and where that energy really comes from.

"Many of these drinks are loaded with caffeine," said Consumer Reports' Jamie Hirsh.

In Canada, the popular Red Bull has been issued a Natural Health Product number, which means it's a safe product if used as directed.

Several other companies have applied for certification. But even certification doesn't mean claims of improved performance and alertness are proven.

And there are legitimate concerns about the amount of caffeine you can consume in these drinks.

Big Buzz boasts the most caffeine with 224 milligrams per 473 millilitre can. Red Bull has about 30 percent less. But the buzz doesn't stop there.

"Many of these drinks contain ginseng, which can amplify the effects of caffeine," warned Hirsh.

Some drinks also contain the additive taurine. Taurine is commonly added to cat food to prevent blindness and heart failure. The health benefit for humans when consumed in a beverage is under study.

Unlike other beverages, there are no calories listed on energy drink labels -- but it doesn't mean they are calorie free.

Some have more than 200 calories per container and you'll pay about three times as much as you would for a can of pop -- or a bottle of water.

"It's okay to enjoy an occasional energy drink. But you should try to choose the low-calorie version to avoid getting too much sugar and calories," advised Hirsh.

And there can be possible side effects from getting your buzz from these products.

Health Canada is investigating reports of electrolyte disturbances, nausea, vomiting and heart irregularities from drinkers.

Most of the energy drinks carry warnings that if you're pregnant, nursing, or sensitive to caffeine, you should avoid these drinks entirely. Health Canada says that's good advice, and warns against mixing these drinks with alcohol - because all that caffeine can mask impairment.

With a report from CTV British Columbia's Chris Olsen.