Caffeine is a drug that stimulates the central nervous system. It makes people more alert, less drowsy, and improves coordination. When combined with certain pain relievers and medicines for treating migraine headache, caffeine makes those drugs work more quickly and effectively. Caffeine alone can also help relieve headaches. Antihistamines are sometimes combined with caffeine to counteract the drowsiness that those drugs cause. Caffeine is also used to treat some breathing problems, because it widens the bronchial airways.

Caffeine was first discovered in coffee in 1820. It is also found naturally in tea and chocolate. Colas and some other soft drinks also contain it. Caffeine is sold in tablets and capsules that can be bought without a prescription. Over-the-counter caffeine brands include No Doz, Overtime, Pep-Back, Quick-Pep, Caffedrine, and Vivarin.

People who use large amounts of caffeine over long periods of time build up a tolerance to it. When that...