Caffeine is the only drug that is widely added to the food supply. Caffeine improves alertness and reaction time in people, whether they're habitual drinkers or not. Up to 3-4 cups of coffee of caffeinated soft drinks are unlikely to cause health problems. Caffeine has been shown to improve mental function and memory and to decrease fatigue. Researchers have found that coffee is quite high in antioxidants. Antioxidants are generally helpful substances, found in foods, which scavenge unstable molecules (free radicals) in your body. Free radicals contribute to oxidative stress, which, over time, can cause inflammation and other unhealthy changes in your cells. However, caffeine is addictive and has been shown to increase blood pressure, particularly in those who are exposed to it infrequently. Drinking the caffeine equivalent of several cups of coffee a day can lead to insomnia, anxiety, and difficulty concentrating.

Additional Resources:

http://www.americanheart.org/presenter.jhtml?identifier=4445
http://www.nlm.nih.gov/medlineplus/news/fullstory_95911.html
http://tuftshealthletter.com/ShowArticle.aspx?rowId=192
http://www.mayoclinic.com/health/caffeine/nu00600/method=print