Energy drinks can have unhealthy short-term affects, such as increased heart rate and increased blood pressure. As of 2014, it's unknown whether energy drinks cause long-term damage to the heart. Energy drinks can also cause dehydration.
Energy drinks contain either caffeine or stimulants that are equivalent to caffeine. The amount of caffeine varies based on the drink. Energy drinks should be avoided when exercising because of the potential for severe fluid loss. They also shouldn't be mixed with alcohol because stimulants can mask the effects of alcohol, causing a person to drink more than he normally would. Alcohol mixed with energy drinks can also cause severe dehydration.