I've already documented the Many Health-Promoting of Drinking Coffee. But is there a dark side? Can drinking all that java and caffeine really be good for you?
As is so often the case, there is always a potential downside and one has to weigh the pros versus cons. Below are some of the things about coffee drinking and caffeine that could get you into trouble:
1) Homocysteine. Coffee is a known increaser of homocysteine, the bad boy of the cardiovascular world. One study found, though, that 200 micrograms of folic acid negated this effect  and so it is possible that a healthy diet with ample B vitamins may keep coffee's homocysteine-raising properties in check. Homocysteine is, by the way, not only a risk factor for heart disease but for erectile dysfunction as well. 
2) Cortisol. Caffeine increases midafternoon cortisol levels.  Of course, cortisol is a muscle-destroying, visceral-fat-building, neuron-killing nightmare.
3) Testosterone to Estrogen Ratio. Interestingly enough, caffeine increases testosterone. Unfortunately, it does not raise testosterone enough proportionately to compensate for the rise in cortisol, leaving one with a lowered testosterone-to-estrogen ratio. 
4) Blood Pressure. There are mixed results here, but one prominent meta-analysis showed that long term caffeine increased blood pressure. 
5) Skin Wrinkling. Many experts feel that caffeine's dehydrating effects may accelerate skin aging and wrinkling.
6) Arrythmia. Some sensitive individuals will experience heart "flutters" while on caffeine. However, it should be noted that, according to one recent meta-analysis, this is a low probability concern 
REFERENCES:1) European Journal of Clinical Nutrition, 2003, 57:1411 1417, "Folic acid supplement decreases the homocysteine increasing effect of filtered coffee. A randomised placebo-controlled study"
2) Psychosomatic Medicine, 2005, 67:734-739, "Caffeine Stimulation of Cortisol Secretion Across the Waking Hours in Relation to Caffeine Intake Levels"
3) Int J Sport Nutr Exerc Metab, 2008 Apr, 18(2):131-41, "Dose effect of caffeine on testosterone and cortisol responses to resistance exercise"
4) Hypertension, 1999, 33:647 52, "The effect of chronic coffee drinking on blood pressure: a meta-analysis of controlled clinical trials"
5) Metabolism, 2006 Dec, 55(12):1564-8, "Hyperhomocysteinemia: a novel risk factor for erectile dysfunction"
6) American Journal of Clinical Nutrition, Mar 2005, 81(3):539-540, "Caffeine and arrhythmia"