Coffee was first introduced in Europe during the second part of the 17th century. It came and it caused controversy with it. Many doctors ( learned ones) said that coffee was a poison and so practice of drinking coffee must be prohibited.
Well, everything is harmful in excess (exception for money, which nobody minds in excess). It have proved to be toxic when tested on lab animals. It can also produce toxic effects in small children. It acts strongly on them and deprive them of sleep retarding their development.
For adults who drink it in moderation, it is definitely not a poison. Most people think that effects of coffee due to caffeine. But this isn’t true. Other substances in coffee are involved too.
THE AROMA: Coffee odour have stimulating effects in various parts of our body. It dilates the blood vessels in brain and improves circulation, removes fatigue toxins from brain
THE HEART: It increases the pulse rate and stimulates heart. I also increase the rate of glycolysis and increases the amount of ATP (energy providing molecule) available for muscle contraction and relaxation. According to one study, caffeine in the form of coffee significantly reduces the risk of heart disease in epidemiological studies. (from Wikipedia)
THE TIME: Coffee produces different effects on different times of the day! In morning, it acts on kidneys and helps to get rid of waste products accumulated at night. After lunch it acts on gastric glands and helps digestion. Afternoon coffee reduces tiredness caused by our hectic morning schedule. And in evenings, it stimulates mind and imagination.
THE OTHERS: It also works on gastric glands ( make them secrete more gastric juice), blood( to remove salts), arteries ( causes stiffness in them) etc etc.
Its all in our hands. Recommendation : Coffee is good. Not more than 4 cups of it is even better.