Caffeine

Caffeine is a central nervous system stimulant and the active ingredient in coffee.

Uses
Caffeine occurs naturally in coffee, tea, cocoa, chocolate, guarana, and many other plants, foods and drinks. Caffeine can also be taken as a supplement and may be added to other food or drinks, especially energy drinks.

Energy drinks
Drinks marketed as energy drinks or sometimes sports drinks typically contain a number of different stimulants, including:
 * high or very high doses of caffeine
 * high or very high amounts of simple sugars
 * taurine
 * guarana extract
 * methionine
 * ginseng
 * other amino acids or natural stimulants

Energy drink brands include such as Spike Shooter, Pimp Juice, Red Bull, and Cocaine.

Evidence
Large doses of caffeine can cause serious adverse reactions, including even fatalities resulting from excessive consumption, especially in the form of "energy" drinks.

Caffeine is mildly addictive, but other substances found in energy drinks are more addictive.

Notable studies

 * 2015, Brain-derived neurotrophic factor concentration may not be depressed in chronic fatigue syndrome - (Abstract)

Costs and availability
Very widely available and inexpensive.

Learn more

 * Caffeine - MedlinePlus
 * Caffine - WebMD
 * Caffeine - Mayo Clinic