Caffeine is a stimulant that increases respiration, heart rate, metabolic rate, and the secretion of certain hormones.
Many persons use it to increase energy and concentration.
Reactions to caffeine are particularly individual. It can cause indigestion, nausea, diarrhea, and dehydration. The body's ability to cool itself is reduced so it is dangerous for hot and humid conditions. There is some scientific suggestion that it may slow calcium absorption in the bones in women and therefore, may be associated with osteoporosis. In elite athletes it is used to alter metabolism, in particular, fats, carbohydrate, and protein use.