Caffeine can be deadly. A lethal dose of caffeine for an average adult is about 10 grams; the equivalent of 100 cups of coffee.
Extreme overdose can result in death. The median lethal dose is estimated to be about 150 to 200 milligrams per kilogram of body mass or roughly 80 to 100 cups of coffee for an average adult. Though achieving lethal dose with caffeine would be exceptionally difficult with regular coffee, there have been reported deaths from overdosing on caffeine pills, with serious symptoms of overdose requiring hospitalization occurring from as little as 2 grams of caffeine. Death typically occurs due to ventricular fibrillation caused by effects of caffeine on the cardiovascular system.