The friend who both pleases and vexes you may be bad for your health. A research collaboration between Brigham Young University and the University of Utah found that mixed-feeling friendships may not let you relax or provide the social support you need during stressful times.

The study is being published today in the Annals of Behavioral Medicine. The researchers found that "unpredictable and ambivalent friendships" raise blood pressure, possibly because they are a source of stress, while more positive friendships help you deal with stress. Next, they hope to study whether those ambivalent friendships can contribute to cardiovascular problems such as high blood pressure or clogged arteries.

That's important information, according to Julianne Holt-Lunstad, an assistant professor of psychology at BYU, because about half of most people's relationships are somewhat ambivalent.

"Our friends can be our best allies and our harshest critics," John Cacioppo, president of the Association for Psychological Science and a University of Chicago professor who was not involved in the research, was quoted in a release about the research. "This research demonstrates that a more sophisticated conceptualization of our social relationships provides richer information about their impact on our health."

The study confirmed an earlier finding that blood pressure is higher when people are with their mixed-feeling friends than with people they either really like or dislike.

For this study, the researchers — led by Holt-Lunstad and the U. psychology department's Bert Uchino, associate professor, and Timothy W. Smith, professor — got about 100 people to each list 10 friends, then answer questions about them, ranging from how long they've known each other and how often they see them to how helpful that person is during times of stress or how supportive, among other questions. Based on that information, the researchers rated the friendships as supportive or ambivalent

They were looking for mid-ground ambivalence, friends who score at least a three out of six in terms of positive feeling but who also are somewhat upsetting.

Then they specified which friend to bring in for the study — either supportive or conflicted — without saying why.

The two were asked some questions in the lab, mostly to get them used to the setting, since it's not where friends normally hang out, Holt-Lunstad said. Then they were separated by a curtain and told to relax and not talk so that the researchers could establish baseline cardiovascular measures.