According to various studies, it is safe to drink three to four cups of black coffee a day. Caffeine, the most widely used psychoactive drug globally, is the main pharmacologically active compound in black coffee. Several factors like the type of coffee bean affect the caffeine content of a cup of coffee. Up to 400 mg of caffeine (roughly the amount of caffeine in four cups of brewed coffee) is recommended as safe for most healthy adults to have per day.