High-caffeine soft drinks may do more than give people a jolt of energy. They may also boost heart rates and blood pressure levels, researchers say.

The results of a small study prompted the researchers to advise people who have high blood pressure or heart disease to avoid energy drinks because they could impact their blood pressure or change the effectiveness of their medications.

The drinks generally have high levels of caffeine and taurine, an amino acid found in protein-rich foods like meat and fish that can affect heart function and blood pressure, the researchers say.

"We saw increases in both blood pressure and heart rate in healthy volunteers who were just sitting in a chair watching movies. They weren't exercising. They were in a resting state," says James Kalus of Henry Ford Hospital in Detroit, who led the study.

The increases did not rise to dangerous levels in the group of 15 healthy volunteers, whose average age was 26, the researchers say.