The American Heart Association reported that drinking up to five 8-ounce cups of coffee per day (approximately 400 milligrams of caffeine) is safe for most people and has been linked to a lower risk of heart disease, heart failure, stroke, and type 2 diabetes, according to the Wall Street Journal. The link between caffeine and heart health is not fully understood, but caffeine’s diuretic effect can lower blood pressure and block a substance that can cause atrial fibrillation, an arrhythmia that can lead to strokes.