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Caffeine (C8H10N4O2), known medically as 1,3,7-trimethylxanthine, is a legal stimulant and diuretic1 used by a large percentage of the Earth's human inhabitants. It can be found naturally in drinks like coffee and tea, and has been artificially introduced into sodas like 'Coke', 'Mountain Dew', and 'Surge'. Additionally, small amounts can be found in solid foods like chocolate. The primary method of obtaining pure caffeine is through the decaffeination of coffee and tea, and in this state caffeine appears as a fine white powder. When ingested, this powder is very bitter.
The Effects of Caffeine on the Human Body
The primary effect of caffeine on the human body, and the one that it is most often consumed for, is that of cardiac stimulation - caffeine gives the consumer an 'energy boost' that lasts for a period of time proportional to the amount of caffeine ingested. Cardiac stimulation is achieved indirectly through the release of epinephrine (adrenaline) by the pituitary gland; however, in order to get the pituitary to release epinephrine, caffeine must first cause an increase in nerve cell activity in the brain.
To do this, caffeine takes the place of adenosine, a naturally occurring chemical in the body that slows down nerve cell activity and dilates blood vessels to allow for the increased oxygen uptake necessary during sleep. Caffeine binds to adenosine receptor sites on nerve cells in the brain, and instead of slowing activity, it quickens the pace at which nerve cells fire, warding off drowsiness. It does this by lowering the trigger level for norepinephrine2, increasing the likelihood that the firing of one nerve cell in the brain will cause neighbouring cells to fire. This increased activity in the brain alerts the pituitary gland, and it releases epinephrine, setting off the 'fight-or-flight' response in the human. One of the functions of the fight-or-flight response is to release sugar (stored in the liver in the form of glycogen) for immediate use; this is the source of the energy used in the 'boost' provided by ingesting caffeine.
How Caffeine Becomes Addictive
In addition to touching off the fight-or-flight response, caffeine also increases dopamine levels in the brain. Dopamine is a neurotransmitter that activates the pleasure centre in the brain; by manipulating the level of dopamine, caffeine artificially makes the consumer feel good for a short period of time. This is the same mechanism used by both cocaine and heroin.
As well as providing an immediate 'feel-good' sensation, caffeine encourages long-term addiction by depriving the consumer of a good night's sleep. Because adenosine reception is essential to deep sleep and it is blocked by caffeine, consumers wake up feeling irritable, and use caffeine to mentally 'awaken' themselves so that they can function 'properly' - as though they had had a good night's sleep. In this way, a positive feedback loop is created, and consumers cannot abstain from caffeine consumption without adverse effects.
Positive Uses of Caffeine
Caffeine can be used in treatment of acute asthma - because of the overall vasodilation3 effect caffeine has on the human body, the airways in the lungs are also widened slightly (this effect is known specifically as bronchodilation), enabling someone afflicted with asthma to breathe more easily. While there are more asthma-specific drugs on the market, caffeine will do if there's nothing else around.
Another use of the vasodilation effect of caffeine can be found in headache relief; constriction of blood vessels in the brain can cause major headache pain, and caffeine relieves this pain by widening the blood vessels. For this reason, caffeine can be found in many specialised headache medicines (for example Excedrin).
Additionally, caffeine can be used to treat ADHD, a hyperactivity disorder. People with ADHD have a lowered ability to focus and a shortened attention span, and stimulants like caffeine (and Ritalin, the most frequently-used treatment for ADHD) allow for longer periods of intense concentration. Unfortunately, the side effects of caffeine make it impractical as a long term solution to most medical conditions.
While caffeine causes negative results when used in the long term, it is - like most things - safe in small amounts; when that paper has to be done by eight tomorrow morning, and there are ten pages left to write, caffeine allows for the presence of mind necessary to write a paper that won't make you wish you had gone to sleep instead of writing it at all.