Through many studies and research that's conducted around the world, caffeine stands out to be one of the main ingredients that provide the boost in most energy drinks available. Thus you might be able to save yourself some money and even a few calories by simply drinking another caffeine-rich alternative, coffee. The caffeine content in a cup of coffee can vary, however, it is generally between 120 milligrams to 230 milligrams in an 8-ounce serving. Similar to coffee, a shot of espresso is also another great alternative as it is very high in caffeine. You'll find around 150 milligrams to 185 milligrams of caffeine in each shot of espresso.