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The number of young adults who smoke both tobacco and marijuana may actually be higher than some recent surveys have suggested, according to a new study.

Researchers from the University of California, San Francisco recruited 18- to 25-year olds through advertisements on Facebook, Craigslist and a survey sampling company and determined how many of them smoked at least one cigarette during the past 30 days. Approximately 3,500 respondents said they did, and they were surveyed again about their use of both tobacco and marijuana.

More than half of tobacco smokers also admitted to smoking marijuana, a higher percentage than what's been reported in other national surveys that utilized interviews to gather data. Those classified as daily smokers were less likely to use marijuana overall, but when they did use it, they used it more often.

In 2009, the U.S. Substance Abuse and Mental Health Services Administration (SAMHSA) National Survey on Drug Use and Health reported that 34.6 percent of past-month smokers age 18 to 25 used marijuana. The new study found that 53.1 percent of past-month smokers in the same age group acknowledged using marijuana.

"That really suggests that social media is a useful mechanism to find young adult smokers and survey them about co-use of marijuana," said Danielle Ramo, a study co-author and post-doctoral scholar at the University of California, San Francisco. "They might be more comfortable reporting their use anonymously online."

Ramo said researchers were careful to ensure that people did not respond multiple times, but acknowledged that using social media sites like Facebook makes it extremely difficult to get a nationally representative sample of respondents.

"This was not random sampling," she said. "These data, even though they showed a large prevalence of use, can't be directly compared to nationally representative surveys."

Despite that limitation, Ramo said the study results suggest that efforts to get young adults to stop smoking should target both tobacco and marijuana, since the use of both substances at the same time is common.

The authors added that the popularity of the internet could make it an effective tool for reaching out to young adults to learn more about their health behaviors and "will likely be instrumental in helping to understand and treat multiple substance use in young adults."