Toxicologist Jan Tytgat (University of Leuven) says that coffee doesn't have a negative influence on your heath. Quite the contrary!
When you drink up to 1 liter coffee a day (4-5 cups) your chances of getting Alzheimers decrease. And because coffee also has a diuretic effect, possible toxic substances leaves your bowel system quickly.
For the rest, coffee has a stimulating effect on your muscles - amond which of the heart. Coffee drinkers have a higher blood pressure and quicker heartbeat. Cafein also widens the blood vessels, and that is why some medicines for the heart contain this substance.
One slight disadvantage: coffee work addictive. Although it's not so that a coffee drinker would commit murder to have his daily cup....