In the last decade, the market for so-called energy drinks has grown rapidly. These beverages are often advertised as nutritious, and some contain vitamins or herbal extracts. But their main ingredients are sugar and caffeine ? and lots of both.  And, Rose Hoban reports,  that can be bad for your health.

Nutritionist Dee Rollins with Baylor University in Texas says she's aware of more than 200 energy drinks that are on the market. She says the amount of caffeine they contain can be amazingly high. "Some of these energy drinks run a couple of hundred milligrams of caffeine per one drink," she says.

But many drink manufacturers don't advertise the amount of caffeine contained in the drinks. So, she warns, people drinking them could easily be taking in too much of this powerful, central nervous system stimulant.

"A small amount increases our awareness, decreases our fatigue, makes us feel a little bit better," Rollins explains. "Generally 250 milligrams a day [is] what would in America be recognized as safe to take. A [level of] 300 milligrams is often considered excessive."

Some energy drinks contain over 200 milligrams of caffeine in one bottle. Rollins worries that when people finish drinking that bottle, they don't realize they've just consumed close to the daily limit of caffeine. Add to that just a single cup of coffee ? containing between 80 to 120 milligrams of caffeine ? and a person can quickly end up over the recommended limit.

Rollins says researchers have found that topping 250 milligrams of caffeine in a day can lead to health problems, and symptoms such as nervousness, headaches, increased heart rate, and higher blood pressure. "The more we take, the more symptoms we might have, depending on the genetics of the person," she explains. "We might have G.I. [gastro-intestinal] irritation ? that would be the gut, the stomach, the intestines; we might have diarrhea. If we take too much we might actually move into depression; insomnia is not uncommon, [nor is] poor concentration."

Too much caffeine has been found to contribute to changes in heart rhythm. Rollins says psychiatrists have also found that caffeine can exacerbate symptoms in people with mental health problems, such as depression, bipolar disorder, anxiety and sleep problems.

Rollins says the way to avoid taking in too much caffeine is to know how much caffeine is in an energy drink, and to drink them in moderation.