500 mg of caffeine at once
That's roughly the amount of caffeine in four cups of brewed coffee, 10 cans of cola or two energy shot drinks. Although caffeine use may be safe for adults, it's not a good idea for children. And adolescents should limit themselves to no more than 100 mg of caffeine a day.Even among adults, heavy caffeine use can cause unpleasant side effects. And caffeine may not be a good choice for people who are highly sensitive to its effects or who take certain medications.eavy daily caffeine use â more than 500 to 600 mg a day â may cause side effects such as: 1  Insomnia. 2  Nervousness. 3  Restlessness. 4  Irritability. 5  Stomach upset. 6  Fast heartbeat. 7  Muscle tremors.