More U.S. college students are making a habit of using marijuana, which has supplanted cigarettes as the smoke-able substance of choice among undergraduates who light up regularly, a study released Tuesday found.
Just under 6 percent of the full-time students surveyed by University of Michigan researchers for the annual "Monitoring the Future" study reported using pot either every day or at least 20 times in the previous 30 days.
By contrast, 5 percent of respondents identified themselves as heavy cigarette smokers, a steep decline from the 19 percent who said they smoked daily in 1999.
