Generally, caffeine, as a stimulating drug, has several positive and beneficial effects on the body, as well as some negative effects; once consumed, caffeine affects the central and peripheral nervous systems, enhancing the activity of dopamine (the "feel good" hormone and neurotransmitter), which reduces stress, anxiety, and fatigue, improves concentration and focus, and increases alertness, motivation, and cognitive and emotional response; according to some studies, caffeine can even increase muscular strength and endurance, as well as speed. Thus, it is often taken by athletes as a safe performance enhancer.
It is notable to mention, however, that caffeine is most effective when taken in moderate doses, as consumption of higher dosages (more than 400 or 600 mg of caffeine a day) can actually increase anxiety and nervousness and may result in abnormal heart rate, increased blood pressure, gastrointestinal problems, headaches, dehydration, and insomnia. Caffeine is not known to cause addiction as some other stimulating drugs do; however, some people may develop a tolerance to its effects; the brain and the body might also become so used to caffeine that not consuming it may cause headaches and nausea.
It takes ten hours for caffeine to completely leave the body. Those who consume it regularly and then abruptly stop may experience caffeine withdrawal, which lasts from two to nine days and is accompanied by symptoms commonly associated with the flu, such as: headache, nausea and vomiting, fatigue, drowsiness, and even muscle pain; it can also cause nervousness, irritability, sleep irregularities, and mood fluctuations.