Medicinal purpose of Caffeine

Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is the world's most widely consumed psychoactive drug.

Unlike many other psychoactive substances, it is legal and unregulated in nearly all parts of the world .Caffeine is a bitter, white crystalline  purine, a  methylxanthine  alkaloid, and is chemically related to the adenine and guanine bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is found in the seeds, nuts, or leaves of a number of plants native to Africa, East Asia and South America, and helps to protect them against predator insects and to prevent germination of nearby seeds. The most well-known source of caffeine is the coffee bean - technically the seed of the  Coffea  plant. People may drink beverages containing caffeine to relieve or prevent drowsiness and to improve cognitive performance.

Uses

Caffeine can have both positive and negative health effects.

It may confer a modest protective effect against some diseases, including Parkinson's disease. Some people experience sleep disruption or anxiety if they consume caffeine.

Caffeine can produce a mild form of drug dependence – associated with withdrawal symptoms such as sleepiness, headache and irritability – when an individual stops using caffeine after repeated daily intake. 

Tolerance to the autonomic effects of increased blood pressure and heart rate, and increased urine output, develops with chronic use (i.e., these symptoms become less pronounced or do not occur following consistent use).

How much is too much?

Up to 400 milligrams (mg) of caffeine a day appears to be safe for most healthy adults. That is roughly the amount of caffeine in four cups of brewed coffee, 10 cans of cola or two "energy shot" drinks.

Keep in mind that the actual caffeine content in beverages varies widely, especially among energy drinks.