According to studies, drinking 2-3 cups of coffee can increase blood pressure as well as the level of a certain stress hormone called cortisol. These stress hormones cause the blood vessels to constrict and the heart to pump harder and could make things worse for those already suffering from hypertension. It has also been observed that drinking caffeinated beverages (such as coffee, tea) when in stress can increase the systolic blood pressure (upper reading of the blood pressure) above 140 mm Hg.
A study was conducted on some students to measure the blood pressure and the cortisol levels after attending 2 ‘low stress’ lectures and 2 stressful exams. Just before the lectures and exams, the students were either given 6 ounces of unsweetened grape juice containing caffeine (equivalent to that present in 2 cups of coffee) or just pure grape juice. It was found that blood pressure increase due to stress alone was 4/1 mm Hg in the low risk group and 7/3 mm Hg in the high-risk group; and 5/4 mm Hg versus 3/3 mm Hg in response to caffeine alone. The combined effect of caffeine and stress showed a significant increase in blood pressure. It increased by 9/5 mm Hg in the low risk group and by 10/6 in high-risk group. In fact, the blood pressure in men in the high risk group shot up to about 140/90 mm Hg or higher and remained high for at least 4 hours.
It was also observed that the combination of caffeine and stress increased cortisol levels by 7.4 nmol/L, which was much more than that caused by stress alone.
According to the researchers the increase in blood pressure during the study was enough to significantly reduce the effect of high blood pressure medicines (antihypertensives) also. Hence it is advisable to avoid caffeinated beverages if one is taking antihypertensives as caffeine can nullify the effect of the medicine, requiring the patient to take higher additional dose of medicine to normalise the blood pressure.
Now that it is clear that caffeine not only increases blood pressure but also reduces the effect of antihypertensives the experts recommend that people who have hypertension or are at risk of developing high blood pressure should avoid taking caffeinated beverages especially when they are under stress.