Caffeine

Caffeine is a central nervous system stimulant and the active ingredient in coffee. For most healthy people, it is not harmful to consume up to 400mg of caffeine a day.

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.

Caffeine is typically used for:
 * increased mental alertness
 * migraines and headaches, including in over-the-counter and perscription drugs
 * improved athletic performance
 * to improve short-term memory
 * obesity/weight loss
 * less commonly, for asthma or gallbladder disease

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.

Caffeine powder
The FDA has issued a warning against the use of caffeine powder, which can provide 1200mg of caffine per 0.15 tablespoons of pure caffeine, this is three times the suggested safe daily limit and a level high enough to cause toxic effects, including seizures.

Perscription drugs
Caffeine is FDA-approved for:
 * migraine and tension-type headaches, when combined pain relief drugs
 * Preventing headaches after surgery (for people who regularly consume caffeine)
 * Caffeine citrate is approved for pauses in breathing that may be followed by low heart rate and low oxygen levels in newborns.

ME/CFS
Many people with ME/CFS use caffeine to improve mental alertness or concentration or to reduce fatigue in the short term, but the 2014 International Association for Chronic Fatigue Syndrome/Myalgic Encephalomyelitis primer cautions against excessive usage due to the risk of tachycardia and agitation. Consuming caffeine in the afternoon or evening is also advised against for ME/CFS patients due to the likelihood of it causing or contributing to insomnia. Adolescents and children with ME/CFS are advised to limit the use of caffeine for concentration/attention to times when it is particularly important, such as for exams, and to be aware of the caffeine found in energy drinks.

Risks and side effects
For most adults, a limit of 400mg of caffeine daily is likely to be safe, although this may not necessarily be safe long term. Just four cups of brewed coffee, ten cans of cola or two typical energy shot drinks provide the maximum recommended amount of 400mg of caffeine. Caffeine sensitivity varies greatly between people so some people will experience harmful effects at much lower doses.

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

Caffeine is mildly addictive, but other substances found in energy drinks are more addictive. Caffeine withdrawal is not considered dangerous.

Excessive caffeine intake causes:
 * Restlessness and shakiness, sometimes called "jitters"
 * Insomnia, especially if drunk/taken in the afternoon
 * Headaches
 * Dizziness
 * Fast heart rate or racing heart
 * Dehydration from increased urination
 * Nervousness
 * Irritability
 * Anxiety
 * Dependency, meaning that higher amounts of it are needed to get the same results
 * Upset stomach or heartburn
 * Reduced calcium absorption
 * Increased blood pressure
 * Muscle tremors

Interactions
Caffeine may interact with certain perscription drugs or other supplements, particularly:
 * Ephedrine, which is used in decongestants
 * Theophylline, a medication used to open up bronchial airways.
 * Echinacea, a herbal supplement sometimes used to prevent colds or other infections

Costs and availability
Very widely available and inexpensive.

Notable studies

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

Learn more

 * Caffeine - MedlinePlus
 * Caffine - WebMD
 * Caffeine - Mayo Clinic
 * Spilling the Beans: How Much Caffeine is Too Much? - FDA