One billion people around the globe use tobacco and 182 million people smoke pot, making these two of the world's most popular drugs, according to the World Health Organization and the United Nations Office on Drugs and Crime.
Many people mix the two drugs together to save money. Tobacco also makes pot inhalation more efficient. This practice, however, may increase the likelihood that users become dependent, the researchers found.
"Cannabis dependence and tobacco dependence manifest in similar ways, so it is often difficult to separate these out in people who use both drugs," said study lead author Chandni Hindocha.
"Cannabis is less addictive than tobacco, but we show here that mixing tobacco with cannabis lowers the motivation to quit using these drugs," added Hindocha, a doctoral student at University College London's clinical psychopharmacology unit.
For the study, researchers examined survey responses from nearly 34,000 marijuana users from 18 different countries in Europe, North and South America, and Australasia who participated in the anonymous online 2014 Global Drug Survey.
Marijuana is consumed in different ways around the world, the study authors said. Mixing marijuana with tobacco is much more popular in Europe than in other parts of the world, the researchers reported July 5 in the journal Frontiers in Psychiatry.
Mixing pot with tobacco is popular with up to 91 percent of European marijuana users, compared to 52 percent of Australian pot users and just 21 percent of New Zealand users.
Tobacco-mixing methods are even less popular in the Americas, where they are used by only 16 percent of Canadian marijuana users, 4 percent of those in the United States, and about 7 percent of Mexican and Brazilian users, the researchers reported.
Use of marijuana vaporizers, which don't use tobacco, was reported by 13 percent of survey respondents in Canada and 11 percent of those in the United States. This method is less popular in other parts of the world, the researchers said.
But, the study authors added, the way in which people use marijuana can affect their motivation to quit or seek professional help to do so.
People who preferred non-tobacco methods of using pot were 62 percent more likely to want professional help to use less marijuana. And they were 81 percent more likely to want professional help to use less tobacco, the findings showed.
"Our results highlight the importance of routes of administration when considering the health effects of cannabis," Michael Lynskey, an addiction specialist at King's College London, said in a journal news release.
"Given a changing legislative environment surrounding access to cannabis in many jurisdictions, increased research focus should be given to reducing the use of routes of administration that involve the co-administration of tobacco," Lynskey added.
The negative health effects of tobacco use are well known. The short-term effects of marijuana use include temporary loss of motor, working memory and decision-making skills. Long-term pot use also may lead to dependence, permanent reductions in brain function as well as heart and lung disease and some forms of cancer, according to the World Health Organization.
The U.S. Substance Abuse and Mental Health Services Administration has more information on tobacco and marijuana.
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