A bad friend -- or lousy roomie -- can really make you sick

Jan. 23, 2012 at 4:18 PM ET

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Psst: Be careful who you befriend. New evidence suggests that a bad run-in with a pal may end up hurting your health.

Gossip, rumors, cat fights, mean girls: We know bad friendships can hurt feelings -- but new evidence is emerging that suggests frenemies can hurt your health, too.

A new University of California, Los Angeles, study links negative social interaction to increased inflammation. Inflammation isn't necessarily a bad thing -- it's an immune system response that fights infections and helps heal physical injuries. But chronic inflammation is tied to an increased risk in developing all kinds of health issues: hypertension, atherosclerosis, coronary heart disease, depression, diabetes and even cancer.

"There’s a lot of research showing that when we are stressed out, it activates all these biological systems" including inflammation, says Jessica Chiang, a UCLA graduate student and lead author of the new report. "And interpersonal stressors are often the biggest stressors people experience in their daily lives." 

The study was published online today in the journal Proceedings of the National Academy of Sciences of the United States of America.

Chiang and her cohorts instructed 122 healthy young adult volunteers to keep nightly diaries over eight days, recording every good, bad and "competitive" social interaction. "Competitive" could mean competing for a love interest's attention, jockeying for position at school or work -- or even a literal competition, like playing soccer. Many of the diary-keepers, Chiang says, used the experiment to vent about that common college complaint: their loud, obnoxious, messy roommates.

Researchers then brought the participants in to assess the amount of cytokine, a proinflammatory protein, in their bodies. They found that the participants who self-reported more negative or competitive social interactions were more likely to have significantly higher levels of the protein.

Chiang notes that for a group of young, healthy students, like the study participants, it's unlikely that the stress caused by bad friendships would lead to any real health effects down the road. But repeated exposure over many years to, for instance, a roomie who refuses to clean the bathroom, can be bad news, especially for those who may be genetically predisposed to heart disease or arthritis, she explains.

Are you surprised that a bad friendship can cause real, physical harm? Discuss this story on our Facebook page. 

Related: Watch two relationship experts tell Kathie Lee and Hoda how toxic friends can wreck your self-esteem.