Caffeine is a chemical found in coffee, tea, cola, guarana, mate, and other products.

Caffeine is most commonly used to improve mental alertness, but it has many other uses. Caffeine is used by mouth or rectally in combination with painkillers (such as aspirin and acetaminophen) and a chemical called ergotamine for treating migraineheadaches. It is also used with painkillers for simple headaches and preventing and treating headaches after epidural anesthesia.

Some people use caffeine for asthma, gallbladder disease, attention deficit-hyperactivity disorder (ADHD), shortness of breath in newborns, and low blood pressure. Caffeine is also used for weight loss and type 2 diabetes. Very high doses are used, often in combination with , as an alternative to illegal stimulants.

Caffeine is one of the most commonly used stimulants among athletes. Taking caffeine, within limits, is allowed by the National Collegiate Athletic Association (NCAA). Urine concentrations over 15 mcg/mL are prohibited. It takes most people about 8 cups of coffee providing 100 mg/cup to reach this urine concentration.

Caffeine creams are applied to the skin to reduce redness and itching in dermatitis.

Healthcare providers sometimes give caffeine intravenously (by IV) for headache after epidural anesthesia, breathing problems in newborns, and to increase urine flow.

In foods, caffeine is used as an ingredient in soft drinks, energy drinks, and other beverages.

People with voice disorders, singers, and other voice professionals are often advised against using caffeine. However, until recently, this recommendation was based only on hearsay. Now developing research seems to indicate that caffeine may actually harm voice quality. But further study is necessary to confirm these early findings.

Caffeine works by stimulating the central nervous system (CNS), heart, muscles, and the centers that control blood pressure. Caffeine can raise blood pressure, but might not have this effect in people who use it all the time. Caffeine can also act like a water pill that increases urine flow. But again, it may not have this effect in people who use caffeine regularly. Also, drinking caffeine during moderate exercise is not likely to cause dehydration.