Energy drinks such as Red Bull, Monster, Rock Star, contain stimulants such as caffeine and guarana, that can be present in amounts far in excess of that in a cup of coffee, a Coke, or a Mountain Dew. Although probably not dangerous if used in moderation and in a user without medical problems such as a cardiac condition or high blood pressure, these drinks should be used carefully because they do elevate heart rate and blood pressure and can cause dehydration.
Energy drinks should not be used while engaging in exercise because they are diuretics and will add significantly to the loss of fluids from sweating, resulting, potentially, in dangerous dehydration with increased body temperature, cramps, muscle spasms, confusion, even seizures and unconsciousness.
Energy Drinks and Alcohol: Caffeinated Alcoholic Beverages (for ex. Four Loko)
Caffeinated alcoholic beverages (CABs) such as Four Loko are cheap, high in alcohol content (12% in some), and contain caffeine and other stimulants. This drink combination is irrational and dangerous because of the stimulants' ability to mask the intoxicating effects of alcohol, leading to severe intoxication, overdoses, and risky behaviors. CABs are even more dangerous if combined with other stimulants such as cocaine, amphetamines, or common ADD medications such as Ritalin or Adderall. Combining a CAB with stimulants can result in serious cardiovascular effects, such as irregular hearbeat and, potentially, a heart attack. CABs, and energy drinks alone, can cause dangerous interactions with physician prescribed medications such as anti-depressants and antibiotics.
More information about this is available in a study summary of energy drinks risks. The study states in part: Stimulants in energy drinks laced with alcohol mask the impairing effects of the alcohol compounding the natural risk taking behaviors of college students and resulting in increased consequences such as driving under the influence, combative behaviors, and sexual assaults.
The research study linked to below showed that college students who drank energy drinks with alcohol were three times more likely to leave the bar drunk, four times more willing to drive drunk, and drank for longer periods of time, than patrons who just drank alcohol. Thombs, et. al. (2010). Event-level analyses of energy drink consumption and alcohol intoxication in bar patrons, Addictive Behaviors, 35 (4), 325-330.