Researchers suggest a rise in marijuana use in the U.S. is down to a reduced perception that the drug is harmful.
Study co-author Dr. Wilson M. Compton, of the National Institute on Drug Abuse (NIDA), and colleagues found that between 2002-2014, marijuana use among adults in the United States increased by almost 3 percent.
The team also identified a 17 percent reduction in the percentage of Americans who perceived smoking marijuana as harmful, perhaps explaining why its use has risen.
However, the researchers identified no increase in prevalence of marijuana use disorders.
While marijuana remains the most commonly used illicit drug in America, as of June this year, 25 U.S. states and Washington, D.C., have legalized the drug for medical and/or recreational purposes.
With these rapidly changing marijuana laws, Dr. Compton notes that it is important to identify trends in marijuana use and dependence across the U.S., and how perception of marijuana risks influences such trends.
"Understanding patterns of marijuana use and dependence, and how these have changed over time is essential for policymakers who continue to consider whether and how to modify laws related to marijuana and for healthcare practitioners who care for patients using marijuana," he explains.
Only a third of Americans think marijuana is harmful
For their study, the researchers assessed 2002-2014 data from the National Survey on Drug Use and Health (NSDUH), involving 596,500 adults from the U.S. aged 18 and older.
Specifically, the team looked at participants' use of marijuana in the past year, the prevalence of marijuana use disorders, and perceptions of marijuana-related health risks.
The results of the analysis revealed that between 2002-2014, marijuana use rose from 10.4 percent to 13.3 percent, and the proportion of adults who initiated marijuana use in the past year increased from 0.7 percent to 1.1 percent in the same period.
Daily or near-daily marijuana use - defined as using the drug at least 5 days a week - also increased between 2002-2014, from 1.9 percent to 3.5 percent.
When it came to perceptions of marijuana risks, the team found that the proportion of participants who perceived once- or twice-weekly use of the drug as harmful to health decreased, from 50.4 percent in 2002 to 33.3 percent in 2014.
The team notes that the rise in marijuana use and changes in perceptions of harm began around 2006-2007.
On generalizing their results to the entire U.S. population, the researchers estimate that the overall number of marijuana users increased from 21.9 million in 2002 to 31.9 million in 2014, while the number of adults who first used marijuana in the past year rose from 823,000 to 1.4 million in the same period.
Additionally, the team estimated that the number of daily or near-daily marijuana users in the U.S. increased from 3.9 million in 2002 to 8.4 million in 2014.
No increase in marijuana use disorders
Despite the increase in marijuana use between 2002-2014, the researchers found that the prevalence of marijuana use disorders among adults in the general population remained stable during this period, at around 1.5 percent.
Among marijuana users, prevalence of marijuana use disorders declined, from 14.8 percent in 2002 to 11 percent in 2014.
Depression, use of tobacco and other substances, a low education, lack of full-time employment, and being male were factors found to increase the risk of marijuana dependence among users of the drug.
The researchers say the high number of new marijuana users might explain why there was no increase in marijuana use disorders.
"We speculate that the many people who have recently (within the past year) started to use marijuana might be using the drug less intensely and have less psychopathology than people who have used marijuana for longer, which could decrease their risk of transition from use to use disorders," say the authors.
Based on their results, the researchers say policymakers in the U.S. might need to target adults' reduced perception of marijuana risks in order to combat abuse of the drug.
"Perceived risk of marijuana use is associated with high frequency of use suggesting the potential value for modifying risk perceptions of marijuana use in adults through effective education and prevention messages."
Dr. Wilson M. Compton
While one key strength of the study is its large sample size, the researchers note that because data was self-reported, it may be subject to bias.
What is more, because the study did not include adults who were homeless, living in shelters, or who were incarcerated, the team says rates of marijuana use and dependence could be higher.
New marijuana laws will 'likely increase marijuana use, disorders'
In an editorial linked to the study, Prof. Wayne Hall, of the University of Queensland in Australia, notes that there has been much concern that increased legalization of marijuana will increase marijuana use and dependence.
While this latest study is not able to conclude that will be the case, Prof. Hall believes it is a likely outcome.
"These changes in the prevalence of cannabis use occurred during a period when many U.S. states legalized cannabis for medicinal use, but before four states went on to legalize recreational cannabis use (after 2014)," he says.
"It is probably too soon to draw conclusions about the effects of these legal changes on rates of cannabis use and cannabis-related harms, but it is likely that these policy changes will increase the prevalence and frequency of cannabis use and, potentially, cannabis use disorders in the longer term."
Prof. Hall adds that large-scale surveys - such as the NSDUH - should continue to monitor marijuana use and dependence in the U.S., in order to determine whether there is an increase due to policy changes.
However, he says these surveys need to address one key limitation: "[...] namely, that they were designed to provide nationally representative samples and do not necessarily provide representative samples of individual states."
"U.S. Federal funding agencies should consider funding oversampling of representative population samples within states that have and have not legalized cannabis for recreational and medical use," he continues.