Caffeine may be the most common drug in the human food supply. That mug of breakfast coffee, the cup of tea in the afternoon and the energy drink or cola on a hot day all contain caffeine.

Because caffeine is a powerful psychoactive drug, it can cause some unpleasant side effects, including sleep disturbances. Caffeine directly stimulates brain activity, even during sleep, which can lead to nightmares. It can also cause or worsen nightmares indirectly through its effects on the quality of your sleep.

Bupropion is a type of medication used to treat depression and help people stop smoking, MayoClinic.com says. Always use bupropion as directed by your doctor and never take more than your prescribed dose to avoid adverse effects.

Caffeine stimulates the central nervous system, providing greater alertness and energy. Drink too much and you may find yourself overly anxious and jittery. Beverages containing caffeine have been enjoyed by mankind for many years, and it does have some beneficial properties. According to MayoClinic.

As a nervous system stimulant, caffeine in coffee, soft drinks and chocolate can make you feel as if someone pressed a "fast-forward" button on your body. While caffeine has many effects, one that is not common is causing you to feel drowsy.

The best-known effects of caffeine include tremors, nervousness and fast heartbeat. However, other symptoms, including nausea and vomiting, are also possible. The recommended amount of caffeine is no more than 300 milligrams per day, according to MedlinePlus.

Various types of dizziness are among the most common complaints that bring people to their doctors, according to MayoClinic.com, but the dizziness of vertigo is in a class by itself.

Nearly 90 percent of Americans consume caffeine, with the principal source being coffee. Chronic coffee consumption has been linked to a substantially lower risk for type 2 diabetes, particularly if you drink five or more cups daily.

Although it is a substance found naturally in plants, caffeine is considered a drug. While most adults can safely consume 2 to 4 cups of coffee daily without feeling any ill effects from the caffeine, combining caffeine with certain medications can be very harmful.

Caffeine is a stimulant found in many foods and drinks that increases energy and alertness.

Fluoxetine, a prescription medication, affects the chemical balance in the brain.

Caffeine is considered a stimulant, meaning it causes a reaction in your body that can speed up your central nervous system, making you feel more awake and alert.