HAVE YOU EVER had a friend who makes plans to hang out but cancels when a better offer comes along? Or a buddy who helped you through a bad breakup, then flirted with your ex? To scientists, these problematic pals are known as ambivalent friends. To a more slang-savvy crowd, they are called frenemies.

Either term has come to describe a range of complicated relationshipsthose that boost you up and bring you down, for any of a variety of reasons. They include the well-meaning friend who is overly competitive, the pal who is a pillar of support when times are tough but cannot quite take pleasure in your successes, and the college buddy who drops everything to lend you a hand when you need one but gossips about you later.

In these troublesome relationships, qualities such as warmth and understanding go hand-in-hand with criticism, jealousy or rejection. Its a friend who drives you nuts, says Karen Fingerman, a psychologist at Purdue University. You love them, you dont want to lose them, but theyre really a pain.

Researchers have only recently begun examining these mixed-emotion associations. So far they are finding that such ties have negative effects on mental and physical well-being, boosting blood pressure and risk of depression while lowering resistance to stress. But if you want to keep your frenemiesand most people doyou can minimize these effects by buffering your interactions with the mixed-weather friends and considering impartial reasons for their hurtful behavior.

Quality over Quantity
Humans are an extremely social species, and a friendless existence has many drawbacks, including depression, hypertension and cognitive decline. But if you want to be happy (and by extension, healthy), having lots of friends is much less important than having good ones. In a 2006 study psychologists Meliksah Demir and Lesley Weitekamp, then both at Wayne State University, gave 423 college students questionnaires about their personality, their happiness level, and the quality and number of their friendships. The researchers defined quality friendships as those scoring high on help, intimacy, self-validation, reliable alliance, emotional security and stimulating companionship. Fifty-eight percent of the variance in happiness could be attributed to the quality of a persons friendships, compared with 55 percent for personality. The number of friends, on the other hand, had no significant effect on how happy a subject was.

From this angle, frenemies are problematic. No friendship is perfect, of course. But frenemies are consistently imperfect, scoring low on factors such as reliable alliance and self-validation, for example. And once you develop ambivalent feelings for a person, future interactions with that person may be judged through that lens, says psychologist Julianne Holt-Lunstad of Brigham Young University. In other words, you are less able to overlook a thoughtless comment made by a frenemy than one made by someone you think of as supportive.

Our lives are riddled with frenemies. From surveys asking people to assess their relationships, Holt-Lunstad and University of Utah psychologist Bert Uchino have found that, on average, about half a persons social network is made up of ambivalent ties. Many are in the family. Fingerman has found that people are likely to view spouses, parents, children and siblings with more ambivalence than friends and acquaintances. One reason: it is much harder to swap out a family member than a friend, no matter how troublesome he or she is. In addition, even irritating family members often provide support and warmth you cannot afford to give up.

Unhealthy Ties
Ambivalent relationships may do more than dishearten. In a study published in 2003 Holt-Lunstad and Uchino asked 102 male and female volunteers to wear blood pressure monitors for three days. Every time a subject had a social interaction lasting more than five minutes, he or she would describe it in a diary and rate the quality of that relationship. Not surprisingly, blood pressure readings were typically higher when individuals encountered ambivalent friends than when they saw supportive friends. But intriguingly, blood pressure was also more elevated in the presence of ambivalent friends than it was with people the subjects disliked but could not avoid (such as classmates or co-workers). You expect very little from someone you loathe, Holt-Lunstad surmises, whereas ambivalent friends, unpredictable as they are, often raise your hopes only to dash them. And that disappointment, or fear of it, can negatively affect your health.

Other research suggests that ambivalent friends can lower resistance to stress. In 2001 Holt-Lunstad and Uchino reported asking 133 individuals aged 30 to 70 to rate important members of their social networks according to how helpful or upsetting they were. Then the volunteers completed two stressful exercises: a mental arithmetic task and a speech defending themselves against a false accusation. The more ambivalent friends a person had, the higher his or her heart rate and blood pressure were, in general, during these activities. The result suggests that supportive relationships buffer the body against stress but that ambivalent friends have the opposite effect. Consistent with that conclusion, the individuals with a greater number of ambivalent friends were more likely to suffer from depression.

If such friends make us unhappy, why do we keep them? In a 2009 study Holt-Lunstad and graduate student Briahna Bigelow Bushman found that people hang onto difficult friendships deliberatelybecause the relationship has a long history, because the good in the relationship outweighs the bad or because, for whatever reason, they just do not want to give up on the person.

Indeed, you may not need to give up on your frenemies if you know how to manage these relationships to minimize the pain they produce. Whether your friend is worth this effort depends on what he or she means to you. But either way, you can work on keeping your end of the friendship bargain. As Holt-Lunstad says, Start with controlling your own behavior and being the kind of friend youd want others to be.

Coping with Frenemies
You can limit the heartache of trouble-somebut valuablefriendships and family ties using a couple of simple strategies. Psychologist Julianne Holt-Lunstad of Brigham Young University recommends avoiding previously problematic subjects or situations. If your frenemy tends to cancel at the last minute, for example, create backup plans. If talking about politics or religion has led to snide remarks, steer clear of that subject.

In addition, give your frenemys motives a positive, or at least neutral, spin. If a friend often calls you at work, you might be tempted to think, She has no respect for my job or my time. But perhaps she is the type who needs to share her news right away. The latter way of thinking is not as personal, explains psychologist Karen Fingerman of Purdue University. Thats the kind of social cognition that contributes to better relationships.