Caffeine the stimulant

Caffeine is a mild stimulant found in coffee, tea, chocolate and many soft drinks. Too much caffeine can cause nervousness and jitters. It may also increase your blood pressure. The amount of caffeine in two to three cups of coffee can raise systolic pressure 3 to 14 millimeters of mercury (mm Hg) and diastolic pressure 4 to 13 mm Hg in people without high blood pressure.

It isn't clear how caffeine increases blood pressure. Some research has found that people who regularly drink caffeine have a higher average blood pressure than those who drink none. Other research has suggested that regular consumers of caffeine develop a tolerance to it — and as a result, caffeine doesn't have a long-term effect on their blood pressure.

In another twist, a 12-year study of 155,000 women found that drinking caffeinated cola may be associated with an increased risk of high blood pressure. However, the same causal relationship was not found with caffeinated coffee. In fact, the study suggested that women who drink caffeinated coffee may actually have a reduced risk of high blood pressure.

Among people who don't consume caffeine on a regular basis, caffeine can cause a temporary but sharp rise in blood pressure. Exactly what causes this spike in blood pressure is uncertain. Some researchers suggest that caffeine narrows blood vessels by blocking the effects of adenosine, a hormone that helps keep them widened. Caffeine may also stimulate the adrenal gland to release more cortisol and adrenaline, which cause your blood pressure to increase.

As a precaution, some doctors recommend limiting caffeine to 200 milligrams a day — about the same amount as in two 12-ounce cups of brewed coffee. Keep in mind that the amount of caffeine in coffee and soft drinks varies by brand. Also, avoid caffeine right before activities that naturally increase your blood pressure, such as exercise, weightlifting or hard physical labor.