Energy drinks usually contain high levels of caffeine and other ingredients , such as taurine, ginseng and carnitine that act as stimulants. In small doses, caffeine is a pretty harmless substance. However, in high concentrations, it can cause sleep disturbances, anxiety, restlessness, elevation in blood pressure and heart rate, abnormal heart rhythms, seizures and even death.Up to 400 milligrams of caffeine a day appears to be safe for most healthy adults. That's roughly the amount of caffeine in four cups of brewed coffee. More than 500 to 600 mg a day may cause side effects such as Insomnia, Nervousness, Restlessness, Irritability, Stomach upset, Fast heartbeat, Muscle tremors. The ingredients that are bad in energy drinks aren't necessarily bad they are just highly concentrated and that's the part that is bad for you.