This relationship was found to be especially strong among young people between the ages of 12 and 17. Nearly a third of traditional cigarette smokers in this age group said they also used cannabis day in and day out, the researchers say. In fact, young smokers were more than 50 times more likely to use cannabis daily than their nonsmoking peers.
Comparing the overall number of daily cannabis users in 2002 and 2014, the researchers saw another trend.
"More people are smoking cannabis on a daily basis," said Renee Goodwin, the lead investigator and an adjunct associate professor at the Mailman School of Public Health at Columbia University. She said all the groups examined -- smokers, former smokers and nonsmokers in the US -- showed increasing rates of daily cannabis use.
Impact of cigarettes
Goodwin and her colleagues collected data from the National Survey on Drug Use and Health for 2002 through 2014, a total of 725,010 people. Respondents, who were offered $30 to participate, answered personal interview questions plus some computer-assisted, self-interview questions designed for privacy and confidentiality.
Participants reported how often they smoked cigarettes, if at all, and how long it had been since their last cannabis use.
Generally, smoking and daily pot use were found to go hand-in-hand.
"Daily cigarette smokers were over seven times more likely to use cannabis daily, and non-daily cigarette smokers were over four times more likely to use cannabis daily than were never-smokers," Goodwin said.
This relationship was strongest for people between the ages of 12 and 17. In 2014, 28% of daily smokers and 13% of non-daily smokers between those ages used cannabis daily, suggesting that about 40% of 12- to 17-year-olds who smoke cigarettes used cannabis daily in 2014.
Some groups showed a stronger association than others. For instance, the link between smoking cigarettes and daily cannabis use was stronger among Hispanic smokers than white smokers and among females compared with males.
Hispanic smokers were more than 10 times more likely to use cannabis daily than nonsmoking Hispanics, while white smokers were more than seven times more likely to use cannabis daily than their nonsmoking peers.
Like Hispanics, female smokers of all groups were found to be more than 10 times more likely to dip into cannabis every day than nonsmoking females. Yet male smokers were more than six times more likely to use marijuana daily than nonsmokers, Goodwin said.
Though previous surveys
have shown declining rates of drug use among teens, Goodwin and her colleagues found an increase in daily use of marijuana in every group.
"The rate of increase was faster among non-daily cigarette smokers vs. daily smokers, and we were surprised to find that the fastest rate of increase of all was among former cigarette smokers," Goodwin said. Looking at the different age groups, she and her colleagues found that "the increase was fastest among those ages 26 and older."
Daily cannabis use also increased among non-daily smokers (3% in 2002 compared with 8% in 2014) and daily smokers (5% in 2002 versus 9% in 2014) and even among former smokers (0.98% in 2002 versus 2.80% in 2014).
Another researcher in the field, Regina A. Shih, a senior policy researcher at RAND Corp. who conducts research on substance use in early adolescence, said the study provides "new information" to health providers.
Shih, who was not involved in the new research, focuses on the underlying factors that explain differences in substance use by race and ethnicity in her own work.
"The fact that this study found a linkage between cigarette smoking and daily marijuana use that was stronger among Hispanic and other non-white respondents compared to white respondents has implications for the US given the changing demographic makeup of the country," she said.
"It was particularly concerning to see that the highest prevalence of daily marijuana use was among the 12- to 17-year-olds who smoked cigarettes," Shih said. It's well-known, she said, that regular marijuana use in teens can have negative consequences for brain health and educational attainment.
Parents might also be troubled by the new research, she said.
Advice to parents
"The study addresses the misconception that families and the public health community don't have to worry as much about non-cigarette smokers," Shih said, noting the overall increase in daily cannabis use, with a faster rate of increase among people who formerly or never smoked cigarettes. The study also suggests the possibility that reductions in daily cigarette smoking are being replaced with an increase in rates of daily marijuana use, she said.
One drawback of the study "is the inability to draw conclusions about what is happening with individual and family, peer or neighborhood factors that can drive the choice among cigarette smokers to use cannabis on a daily basis," she said.
Research will need to examine the reasons adolescents -- particularly younger ones -- to use cannabis daily in addition to or instead of cigarettes.
"With the changing landscape of recreational legalization by states, it will be important to continue to monitor time trends in daily marijuana use by medical versus recreational use and modes of use," Shih said.
, a research professor at the University of Michigan Institute for Social Research, agreed that trends in marijuana use are of particular interest today "because of the growing wave of states that are legalizing marijuana for recreational use."
Miech, who was not involved in the new study, said there's some concern that legalization could increase marijuana use among underage people. "Legalization has the potential to increase marijuana use in all states," he said, "because many will interpret legalization as a signal that marijuana use is safe and state-sanctioned."
The study also suggests a new twist on the "gateway" theory that using cigarettes puts people at risk of using marijuana, Miech said: "It is possible that, moving forward, some people may start first using marijuana and then transition to cigarette smoking."
This "reverse gateway" effect hasn't been seen much, he said, and vaping might also lead to additional "transitions between cigarettes and marijuana use."
Goodwin said the issue is "complicated," especially for parents.
"There may be a lot of confusing messages today among youth today about whether and to what degree cannabis use is harmful, especially in comparison with cigarettes," she said. "In many places, youth are clear that cigarettes are harmful, but with marijuana especially amidst rapid legalization, this is not clear.
"What we are talking about here is daily cannabis use, and that is generally agreed upon to have a number of potential risks," including physical, neurological and mental, Goodwin said.
"Cigarette smoking -- which is sometimes easier to detect -- is a strong indicator of increased likelihood of cannabis use," she said. Talking to kids about the potential risks of both cigarettes and cannabis is "critically important at an early age."