Upto 400mg of caffeine in a day appears to be safe. That amounts to roughly 4 cup of coffee.an adolescent should restrict caffeine intake to 100mg. even in some adults, caffeine can cause certain side effects. If caffeine taken in amounts as high as 500-600mg one can suffer from insomnia, nervousness, restlessness, irritability, stomach upset, fast heartbeat, muscle tremors. Some people may be more sensitive to coffee, as much as 1 cup could also trigger any of these symptoms. Some medications and herbal medications may interact with caffeine. Thus when on medications, talk to your doctor or pharmacist whether caffeine intake might affect your medications.