Pharmacologically, caffeine is classified as a mild stimulant. In addition to "waking up" the central nervous system, caffeine stimulates increased production of stomach acid, encourages the kidneys to produce urine, briefly, speeds up heart rate and gives a slight boost to athletic performance. In high doses, caffeine can cause rapid heartbeat, irritability, insomnia and anxiety in some people.
Caffeine is quickly absorbed into the body and, depending upon individual differences and the amount consumed, reaches your body cells within 15 to 45 minutes. How long it takes your liver to break down caffeine and your kidneys to excrete it varies from person to person because over time, your body becomes conditioned to the amount you drink. In general, caffeine has a half-life of approximately 3 hours (meaning that within 3 hours time 100 milligrams is reduced to 50 milligrams for instance). If you consume caffeine regularly (in coffee or other beverages), you will develop tolerance to the substance. Many people, however, become addicted to caffeine and find that cutting down after long use causes withdrawal symptoms such as headaches, irritability and fatigue within 18 to 24 hours. In the past, there have been reports of an association between caffeine and heart disease, certain types of cancer, infertility and other medical problems. Actual scientific evidence to support these claims is lacking. Women who are pregnant or are trying to get pregnant, as well as people who experience irregular heartbeats or palpitations, are often advised to cut back on or eliminate caffeine-containing substances. But moderate doses of caffeine, or the equivalent of up to three cups of coffee a day, are considered safe for most people.
A pharmacologically active (drug-like) dose is 100 milligrams (mg). A 5-ounce cup of brewed coffee contains between 110 and 150 mg, a cup of instant coffee has 64 to 124 mg, a cup of brewed tea has between 20 and 90 mg, and a 12-ounce glass of cola has about 60 mg. A glass of chocolate milk has very little caffeine-only about 7 mg.