Many studies have been conducted to explain the relationship between coffee, caffeine and hypertension. They are largely in agreement that caffeine does slightly raise blood pressure in the short run and that over time individuals who consume large quantities of coffee are more likely be diagnosed with hypertension. Two studies, one published in The Annals of Pharmacotherapy and the other in the American Journal of Clinical Nutrition, offer more details.

The Annals of Pharmacotherapy

These researchers found that the effects of caffeine on blood pressure occur about 30 minutes after intake, have their maximum effect at 1-2 hours and have a lingering effect for 4 or more hours. Also, at its peak caffeine was found to raise systolic blood pressure 3-15 mm Hg and diastolic 4-13 mm Hg. This report concludes that caffeine consumption should be considered when interpreting blood pressure readings. (Mort 2007)

American Journal of Clinical Nutrition

Over 24,710 Finnish patients were tracked for an average of over 13 years to see if there was a link between coffee consumption and antihypertensive drug treatment. They found that while coffee drinking does increase the risk of being prescribed hypertension drugs, the increased risk for those who drink about 1 cup of coffee a day was marginally significant.(Gang 2007)

SOURCES:

Gang Hu, Pekka Jousilahti, Aulikki Nissinen, Siamak Bidel, Riitta Antikainen and Jaakko Tuomilehto(2007). Coffee consumption and the incidence of antihypertensive drug treatment in Finnish men and women. American Journal of Clinical Nutrition, Vol. 86, No. 2, 457-464, August 2007. Retrieved September 23, 2008 from http://www.ajcn.org/cgi/content/full/86/2/457.

Mort, Jane R., Kruse, Heather R. (2007). Timing of Blood Pressure Measurement Related to Caffeine Consumption. Published Online, 19 December 2007, www.theannals.com, DOI 10.1345/aph.1K337.
The Annals of Pharmacotherapy: Vol. 42, No. 1, pp. 105-110. DOI 10.1345/aph.1K337. Retrieved September 23, 2008 from http://www.theannals.com/cgi/content/abstract/42/1/105.