The types of "energy drinks" in grocery stores are not likely to be dangerous as long as you do not drink them regularly. However, they are not a healthy way to boost your energy. The two "active" ingredients that are most often included in energy drinks are sugar (in various forms) and caffeine.
Sugar and other sweeteners, such as high-fructose corn syrup, affect your metabolism. Getting too much sugar in your diet can raise blood pressure, worsen cholesterol levels, add to obesity, and shift your metabolism towards a diabetes state. This effect goes beyond the calorie content of sugar. It relates to the way sugar is digested and processed in your body.
Caffeine is safe for most people, in modest doses. Extra caffeine is not recommended during pregnancy. Caffeine can cause side effects such as tremors, anxiety, insomnia, or heart palpitations (skipped beats, extra beats, or a racing heart). If you drink caffeine regularly, you can have withdrawal symptoms, such as strong headaches, if you miss your drink.
Several other ingredients that are sometimes included in "energy drinks" could cause problems if you consumed the drinks regularly. These include creatine (which can cause kidney damage if you are exposed to a large amount of it) and gingko biloba, which can interact with medicine to thin the blood and lead to bleeding.
Some people react to "energy drinks" with allergic reactions or nausea.