You could fill a store with all the energy drinks now available. They promise to make people feel more energetic and think more clearly. These products have names like Red Bull, Monster, Ripped Force, Speed Stack and 5-Hour Energy. They appeal mainly to young people and are fueled mainly by caffeine.

Chad Reissig at Johns Hopkins University in Baltimore, Maryland, is a medical researcher who studies drug dependence. He says the team he works on got interested in studying energy drinks because of the explosion in their popularity in the last few years.

He says the researchers found three major things. One: there are hundreds of brands of energy drinks in most major countries. Two: the drinks are not clearly labeled with enough information. And three: the amount of caffeine varies greatly. Some contain as little as fifty milligrams, others as much as five hundred.

By comparison, a cup of Starbucks brewed coffee contains three hundred thirty milligrams in four hundred seventy-three milliliters.

Some energy drinks contain a mixture of ingredients listed as an "energy blend." Ingredients like taurine, guarana and inositol are natural substances. But Chad Reissig says scientists do not know a lot about them and how they interact with each other and caffeine. And worse, he says, there is no listing of the amount of each ingredient.

Some people combine energy drinks with alcohol. They think they can drink more alcohol that way and not be affected. Studies, however, suggest that they are still under the influence of the alcohol even if they do not feel that way.

Some makers of energy drinks do provide warnings. For example, a popular energy shot warns against use by people who are pregnant, nursing a baby or under the age of twelve. Energy shots are small bottles of liquid. The directions also advise people to limit caffeine products and to drink only half the bottle if they want a moderate amount of energy.

The team at Johns Hopkins University School of Medicine published a report in the journal Drug and Alcohol Dependence. They suggested better labeling with the amount of caffeine and other ingredients clearly listed on the drink. The researchers are continuing to study energy drinks. They are currently seeking young people who have had a bad experience after drinking them.