Caffeine may cause a short, but dramatic increase in your blood pressure, even if you don't have high blood pressure. It's unclear what causes this spike in blood pressure. The blood pressure response to caffeine differs from person to person.
Some researchers believe that caffeine could block a hormone that helps keep your arteries widened. Others think that caffeine causes your adrenal glands to release more adrenaline, which causes your blood pressure to increase.
Some people who regularly drink caffeinated beverages have a higher average blood pressure than do those who drink none. Others who regularly drink caffeinated beverages develop a tolerance to caffeine. As a result, caffeine doesn't have a long-term effect on their blood pressure.
If you have high blood pressure, ask your doctor whether you should limit or stop drinking caffeinated beverages. The U.S. Food and Drug Administration says 400 milligrams a day of caffeine is generally safe for most people. However, if you're concerned about caffeine's effect on your blood pressure, try limiting the amount of caffeine you drink to 200 milligrams a day — about the same amount as is generally in two 8-ounce (237-milliliter) cups of brewed coffee. Keep in mind that the amount of caffeine in coffee, energy drinks and other beverages varies by brand and method of preparation.
Also, if you have high blood pressure, avoid caffeine right before activities that naturally increase your blood pressure, such as exercise, weightlifting or hard physical labor.
To see if caffeine might be raising your blood pressure, check your blood pressure before drinking a cup of coffee or other caffeinated beverage and again 30 to 120 minutes afterward. If your blood pressure increases by about 5 to 10 points, you may be sensitive to the blood pressure raising effects of caffeine. If you plan to cut back on caffeine, do so gradually over several days to a week to avoid withdrawal headaches.