One of the world's most popular drugs, caffeine is a stimulant that affects the central nervous system, the digestive tract, and the metabolism. Caffeine is found in coffee beans, tea leaves, cocoa beans, and products derived from these sources. It is absorbed quickly in the body and can raise blood pressure, the heart rate, and brain serotonin levels (low levels of serotonin cause drowsiness). Withdrawal from caffeine can cause headaches and drowsiness.

The pharmacological active dose of caffeine is defined as 200 milligrams, and the daily recommended not-to-exceed intake level is the equivalent of one to three cups of coffee per day (139 to 417 milligrams).

You get caffeine — and more than you think — in numerous ways. A brewed cup of coffee has about 139 mg of caffeine, a brewed cup of tea (not herbal) has 48 mg of caffeine, one cup of semisweet chocolate chips has 92 mg, and one ounce of bittersweet chocolate has 18–30 mg.

If you feel nervous or jittery, try to find a balance between how much caffeine helps you focus, and how much leaves you on edge.