What is considered high caffeine intake in medical terms? how many milligrams?

There isnt consensus: It is highly variable. Many people can drink 10-15 cups of coffee per day (about 60-80 mg of caffeine in average cup) and get absolutely no side effects. Other people have one cup and can't sleep that night, are jittery and even get a tremor. For most people, 2-3 cups of coffee in the morning (or other caffeinated drinks), somewhere between 150-200 mg of caffeine, is well-tolerated, not harmful.