A small cup of coffee or other limited amount of caffeine doesn't have any affect on your blood pressure readings, although larger amounts can under certain circumstances. The amount of caffeine that can affect your reading is about 200 mg to 300 mg, the Mayo Clinic website notes, or the amount found in two or three cups of coffee. Although a change in blood pressure is possible, it is not always inevitable.
Normal Blood Pressure
If you have normal blood pressure, caffeine can increase it noticeably, but the increase is temporary and only lasts a short time. The increase generally includes the systolic and diastolic pressure readings, increasing each by approximately 4 to 13 mm Hg, or millimeters of mercury, according to the Mayo Clinic website. That means if you have a normal blood pressure reading of 120 mm for your systolic pressure and 80 mm for your diastolic pressure, caffeine can increase it up to about 133 over 93.
High Blood Pressure
If you have high blood pressure to begin with, the chances of caffeine increasing it are even greater, according to Columbia University Health Services' "Go Ask Alice" column. While the column did not mention how much higher your blood pressure can soar if you already suffer from hypertension, it did note that the increase was much greater than for those who usually have normal blood pressure readings.
Explanation
No one is sure why caffeine can increase blood pressure, although the Mayo Clinic site reports a few theories. One is that caffeine may cause your arteries to constrict by blocking a hormone that usually keeps them wider. Another is that caffeine may boost your adrenal glands' release of more adrenaline, with the greater amount of adrenaline leading to the increase. In either case, the increase is always temporary rather than resulting in long-term increase in pressure.
Considerations
Coffee is not the only food or beverage that contains caffeine and, therefore, the ability to increase your blood pressure. Sodas, chocolate, tea and other soft drinks, such as energy drinks, can also contain varying amounts of caffeine. If you already suffer from hypertension, cutting down on your caffeine intake can help keep your blood pressure from soaring even higher. Drinking a cup of coffee or otherwise consuming caffeine before a blood pressure test can also artificially raise your blood pressure and result in inaccurate readings on the test.
