In moderation -- different for everyone, but generally considered to be 1-3 cups daily -- coffee can be good for you. It's linked to a reduced risk of type 2 diabetes, Parkinson’s disease and Alzheimer’s, and may even reduce the risk of certain types of cancer. (These are still associations -- the research isn’t yet definite on coffee’s effects.)
But too much regular coffee (containing caffeine) can be harmful. Keep in mind, caffeine is a central nervous system stimulant. You could experience psychological effects such as anxiety, nervousness and restlessness, as well as health consequences such as an increase in LDL (“bad” cholesterol), high blood pressure, heart palpitations, loss of bladder control, reflux (heartburn), and insomnia. The most serious effects: heart attack, spontaneous abortion and reduced fetal growth.
A cup of coffee contains about 100 to 150 mg of caffeine (a cup of black tea has about half that amount). For most people, 200 mg of caffeine per day -- the amount in 1 ½ to 2 cups of coffee -- poses no risk. (That's also the suggested upper limit for pregnant women on the American Congress of Obstetricians and Gynecologists website.) However, if you don’t habitually take in caffeine, even 100 mg of caffeine, could trigger some effects, although it’s doubtful they would be serious.
Although health authorities don’t have an agreed-upon “safe” level, a webpage on the Food and Drug Administration recommends an upper limit of 400 mg to 600 mg. And while deaths from caffeine are rare, they do occur at levels starting about 5,000 mg (25 to 50 cups of coffee). Powdered caffeine can deliver that dose in just a teaspoon or two -- and it’s caused a number of deaths, so avoid this substance.