The worst influence in a teen's life might not be the pothead kid down the street. A new study from USC says the bad influence might be social media itself.

Teens who see photos of friends smoking and drinking on Facebook and other social media sites are more likely to smoke and drink themselves, says the study published in today's online edition of the Journal of Adolescent Health.

Researchers surveyed 1,563 tenth-graders from the El Monte Union High School District in October, 2010 and April, 2011. They were asked about their social media use and about possible drinking and smoking.

According to the study, about one-third said they had a friend who was a drinker or smoker. Half had tried alcohol and about 30 percent smoked. About half used Facebook regularly.

Interestingly, Facebook users were less likely to be Latino and less likely to smoke or drink compared to MySpace members.

A summary of the research says:

Exposure to friends' online pictures of partying or drinking, however, was significantly associated with both smoking and alcohol use. Teens whose close friends did not drink alcohol were more likely to be affected by increasing exposure to risky online pictures.

Thomas W. Valente, professor of preventive medicine at the Keck School of Medicine of USC and the study's principal investigator, says:

Our study shows that adolescents can be influenced by their friends' online pictures to smoke or drink alcohol. To our knowledge, this is the first study to apply social network analysis methods to examine how teenagers' activities on online social networking sites influence their smoking and alcohol use.

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