Increased Blood Pressure. The combination of caffeine and ephedrine can increase your blood pressure. Ephedrine and caffeine stimulate your sympathetic nervous system by increasing neurotransmitters, such as norepinephrine. This stimulant effect can increase your blood pressure by constricting your blood vessels. This is why many diet pill formulas, often called an ECA stack, contain ephedrine, caffeine and aspirin to thin your blood to offset the blood vessel constriction.