Do the Ingredients in Energy Drinks Work?

Caffeine

What is it?: A chemical compound that stimulates your central nervous system. Most energy drinks contain between 140 and 170 milligrams (mg) of caffeine in a 15- or 16-ounce can.

Does it work?: Java junkies certainly think so. As for the science, an Austrian study showed that men who swallowed 100 mg of caffeine had a bigger boost in brain activity after 20 minutes than those who took a placebo.

Plus, a new University of Chicago study found that a 200 mg jolt made fatigued people feel twice as alert as noncaffeinated participants. "Caffeine indirectly affects many different neurotransmitters," says Andrew Scholey, Ph.D., an herb and nutrition researcher at Australia's Swinburne University of Technology.

Is it safe?: The most caffeine-packed energy drink contains the equivalent in caffeine of about two 8-ounce cups of coffee. If downing that much joe doesn't make you jittery, then quaffing a can shouldn't pose a problem. Of course, if you combine that with other caffeinated beverages throughout the day, then the sum total stimulation could cause headaches, sleeplessness, or nausea.

On the other hand, if you're not a regular coffee or cola drinker and you battle high blood pressure, the occasional energy drink could be trouble. Researchers in Finland reported that the caffeine in two to three cups of coffee can cause BP to spike by up to 14 points.
http://www.menshealth.com/mhlists/effectiveness_of_energy_drinks/Caffeine.php