Caffeine can increase blood pressure by causing the blood vessels to narrow and the heart to beat faster. This effect can be especially pronounced in people who already have high blood pressure.
While the effect of caffeine on blood pressure varies from person to person, it's generally recommended that people with high blood pressure limit their caffeine intake. Drinking too much caffeine can also cause dehydration, which can further raise blood pressure.
For these reasons, it's a good idea to talk to your doctor about how much caffeine is safe for you to consume if you have high blood pressure.