Half of young adult tobacco smokers also have smoked marijuana in the last 30 days, according to a recent Facebook-based survey conducted by UCSF researchers, indicating a greater prevalence of marijuana and tobacco co-use among smokers age 18-25 than previously reported.

Other recent studies have shown that approximately 35 percent of young adult tobacco smokers used marijuana within the last month.

“We were curious whether rates would be different in our study where we reached out through social media and the Web,” said lead author Danielle Ramo, PhD, a post-doctoral scholar in the UCSF Department of Psychiatry. “And rates were much higher, which shows the problem might be larger than we realize.”

Survey participants were recruited solely online, a departure from traditional surveys that rely on face to-face interviews, phone interviews or completing questionnaires. The UCSF researchers primarily used Facebook through a series of paid advertisements, in addition to Craigslist and a survey sampling company to reach out to young adults. The results, the researchers said, indicate young adults might be more inclined to answer honestly via anonymous online sampling.

The research will be published in Addiction Science and Clinical Practice on April 18.

The first phase of the survey was used to identify tobacco smoking patterns only. A second stage asked participants to answer the tobacco and marijuana use survey, which employed data encryption to ensure anonymity and prevent multiple entries. Of the 3,500 individuals who completed the marijuana and tobacco co-use survey, usage was highest amongst Caucasians, people from the Northeast, people in rural areas and among the non-student population. Of the 68 percent who were daily smokers, 53 percent had used marijuana in the last month.