Caffeine is a very powerful drug with quick and definite effects on the central nervous system, the peripheral nervous system and the gut. It is used by many prescription and over-the-counter pills. It is also a habituating drug. You need to take more and more to experience the same effects. If coffee and tea were discovered today, the FDA would most likely not release them to the public.

Caffeine stimulates the release of fatty acids into the blood. With more fatty acids available, muscles need less glucose. A few years ago it was quite popular for marathon runners to take a lot of caffeine before a marathon, hoping that the extra fatty acids would provide them with extra fuel, saving their limited glycogen supply. The caffeine did indeed seem to help in the beginning, but it seemed to exacerbate other long-distance running problems. One draw back is that caffeine is a diuretic and promotes water loss from the body. Caffeine on a hot day increases the risk of dehydration and heat exhaustion.

Another problem with taking caffeine for endurance events is that is competes with insulin. Many runners go on high carbohydrate diets and eat a lot of sugary food and drinks in order to maintain muscle glycogen. The resulting high blood sugar keeps blood insulin levels high, which inhibits the release of fatty acids from fat cells. Caffeine stimulates the release of fatty acids, and insulin inhibits it.

Runners are then faced with a choice of whether they should increase fatty acid levels by drinking coffee or increase glycogen levels by having some carbohydrate-rich donuts. Caffeine also contributes to tension and nervousness just before a race. It can also cause an irritable bowel. This combination can be detrimental to a runner's career.

Caffeine consumption has also been linked with cancer. In 1981, the Harvard School of Public Health suggested that more than half of 20,000 cases of pancreatic cancer occurring in the United States each year may be linked to coffee drinking. According to the study, even people drinking as little as a couple of cups a day were at risk. Five years later, the exact same research group reversed their findings, saying that no link existed between pancreatic cancer and caffeine consumption. Breast cancer and bladder cancer have likewise been shown to have absolutely no connection to caffeine consumption.

Other studies have created some fear in the public with claims that caffeine increased the risk of heart disease, raised cholesterol levels or caused fibrocystic breast disease. After which new studies revealed that the claims had no validity.

More recent caffeine studies have also shown to have a positive effect on people who suffer from bronchial asthma. Long term moderate consumption of coffee in the amount of one or two cups a day eases the symptoms bronchial asthma sufferers experience. Furthermore studies show that in some cases, a prevention of a recurring asthma attack is caused by drinking coffee. In a survey of 70,000 asthmatic adults who drank three or more cups of coffee a day experience 27 percent fewer asthma attacks than those who did not drink coffee.