A bit of both really!
- It contains caffeine which is a stimulant
- It is acidic so can cause discomfort to people with digestive complaints
However:
There was a very interesting article from a published paper called 'Coffee, Cirrhosis, and Transaminase Enzymes' in The Archives of Internal Medicine. Researchers found that a typical serving of coffee contains more antioxidants than typical servings of grape juice, blueberries, raspberries and oranges. These same anti-inflammatory properties may explain why coffee appears to decrease the risk of alcohol-related cirrhosis and liver cancer.
According to Rob van Dam, a Harvard scientist and the lead author of The Journal of the American Medical Association review:
“I wouldn’t advise people to increase their consumption of coffee in order to lower their risk of disease,” Dr. van Dam said, “but the evidence is that for most people without specific conditions, coffee is not detrimental to health. If people enjoy drinking it, it’s comforting to know that they don’t have to be afraid of negative health effects.”
It should be noted that freshly ground coffee is healthier than instant coffee, which is stripped down and processed, and because it is a stimulant it will interrupt healthy sleep patterns if taken late in the day. But apart from that, enjoy it!