Scientists claim to have found evidence that cigarettes are a gateway drug to marijuana.
US researchers randomly selected incoming college students from two universities - one in the Northwest and one in the Midwest - to participate in the study.
Students were interviewed prior to entering college and again at the end of their freshman year regarding their attitudes, intentions and experiences with substances.
Students were asked if they had used tobacco or marijuana ever in their lives and in the past 28 days. Researchers also assessed the quantity and frequency of marijuana and tobacco use in the past 28 days.
"Contrary to what we would expect, we also found that students who smoked both tobacco and marijuana were more likely to smoke more tobacco than those who smoked only tobacco," said study author Megan Moreno, an investigator at Seattle Children's Research Institute and associate professor of pediatrics at the University of Washington.
Results showed that prior to entering college, 33 per cent of the 315 participants reported lifetime tobacco use, and 43 per cent of lifetime users were current users.
In addition, tobacco users were more likely to have used marijuana than those who did not use tobacco.
By the end of their freshman year, 66 per cent of participants who reported tobacco use prior to entering college remained current users with an average of 34 tobacco episodes per month.
Of these, 53 per cent reported concurrent marijuana use. Overall, users of both substances averaged significantly more tobacco episodes per month than current users of tobacco only (42 versus 24).
"These findings are significant because in the past year we have seen legislation passed that legalises marijuana in two states," Moreno said.
"While the impact of these laws on marijuana use is a critical issue, our findings suggest that we should also consider whether increased marijuana use will impact tobacco use among older adolescents," Moreno added.
The research was presented at the Pediatric Academic Societies (PAS) annual meeting in Washington, DC.
