Caffeine was once classed as a banned substance but was removed from the World Anti-Doping Agency prohibited list in 2004. Caffeine is a stimulant and has pharmacological action on the body therefore is classed as a drug rather than a nutrient.
Caffeine is found in everyday drinks and foods, such as coffee, black tea, green tea, cola, herbs such as guarana, both dark and white chocolate and it is added to a number of energy drinks and sports drinks and gels. 
Caffeine acts on the central nervous system, increasing alertness and concentration. These effects reduce the perception of fatigue and allow exercise to be maintained at a higher intensity for a longer period. 
Large number of studies show that caffeine intake can enhance performance at doses of 1-3mg/kg. There appears to be little increase in performance above 3mg/kg. for a 70kg person, this would be 210mg = 2 cups of coffee or 2 cans of caffeinated energy drinks.
Performance benefits occur soon after consumption, so caffeine may be consumed just before exercise, spread throughout exercise or late in exercise as fatigue is beginning to occur.
Although caffeine is a diuretic, a daily intake of less than 300mg caffeine results in no larger urine output than water. Taking caffeine regularly builds up your caffeine tolerance so you experience smaller diuretic effects.
Caffeine consumption likely causes a short-term spike in blood pressure after consumption, with the spike being more pronounced in those who don’t normally ingest caffeine and in those with hypertension. However, the evidence concerning the long-term effects of caffeine and caffeinated beverages on blood pressure is mixed.
Caffeine may raise eye pressure, but only in those who have pre-existing eye conditions like glaucoma.
Coffee contains oily substances called diterpenes, and the 2 main types are cafestol and kahweol that increase the cholesterol levels. Coffee filters trap most of the cafestol and kahweol though, so instant or filter coffee don’t contain much of these, only boiled coffee.