Research has shown marijuana legalization doesn’t increase teen use, but a new study concludes the laws may be increasing abuse of the drug among adults.

Marijuana legalization laws don’t appear to increase the use of the drug among teenagers.

However, medical marijuana laws may be driving up the use and abuse of the drug among adults.

That’s the basic conclusion of a study published today that has reignited the debate over whether marijuana laws encourage illegal use of cannabis.

While the researchers don’t take any position on whether medical marijuana should be legal, they say this is an important topic to study.

“It’s important to think about if these laws can have adverse health impacts,” Deborah S. Hasin, PhD, professor of clinical epidemiology at Columbia University Medical Center, and lead author of the study, told Healthline.

Reaction was swift from those who support marijuana legalization laws.

“America’s real-world experience with medical marijuana regulation … finds that cannabis can be legally produced and dispensed in a responsible manner that positively affects the lives of patients but does not inadvertently or adversely impact overall public health or public safety,” Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws (NORML), told Healthline.

Read more: Is the acceptance of marijuana reaching a tipping point? »

Medical marijuana use is now legal in 29 states and Washington D.C.

In eight states, recreational marijuana use is legal for adults.

For their study, the researchers used data from three different time periods.

The first was 1991-1992, when marijuana was not legal in any state.

The second was 2001-2002, when six states allowed medical marijuana use.

The third was 2012-2013, when 15 states had legalized medical marijuana use.

The researchers analyzed responses to national surveys from nearly 120,000 adults from 39 states.

During the time periods, the rates of illegal marijuana use increased in all 39 states.

The researchers said that in states that haven’t legalized medical marijuana, the rate of illegal use among respondents rose from 4.5 percent to 6.7 percent during the time period.

In states that allow medical marijuana use, the rate of illegal use increased from 5.6 percent to 9.2 percent. That’s a higher use rate and a bigger percentage increase.

The researchers defined illegal use as any use of marijuana that didn’t conform with the laws within the respondents’ state.

The researchers also concluded that in states without marijuana legalization laws, the rate of marijuana use disorders went from 1.3 percent to 2.3 percent during the time period studied.

In states with medical marijuana laws, the disorder rate jumped from 1.5 percent to 3.1 percent. Again, a higher rate and a bigger percentage increase.

Hasin said although the percentages are all single-digit numbers, the impact could be serious if the figures are extrapolated into the entire population of the United States.

“It can translate into some pretty big numbers,” she said.

The study was different from others in that it surveyed only adults and not teens.

The results were somewhat similar to a 2016 RAND Corporation study that concluded adults who use marijuana for medical purposes are more likely to consume or vaporize the drug than recreational users.

Read more: If marijuana is medicine, why can’t we buy it in pharmacies? »

The conclusions from this latest study, however, are significantly different than other studies on marijuana laws and teen use of the drug.

Most recent research has concluded that marijuana legalization laws do not prompt an increase in use of the drug by teens.

Those studies include one conducted in Colorado before and after that state passed its medical marijuana law.

It also includes research published in January that concluded there was no relationship between adolescent use of marijuana and state laws legalizing the drug’s use for adults.

The differences between the age groups is important to a number of experts because of research showing marijuana having negative impacts on the developing brain of people under the age of 25.

These studies prompted the American Academy of Pediatrics to issue a warning in February to parents saying marijuana can be harmful to children.

Nonetheless, the question still remains as to why marijuana laws seem to have no impact on teens but do encourage increased use by adults.

Hasin said she may have an explanation.

She told Healthline it’s possible the laws don’t influence teens because the drug remains illegal for them to use even in states where recreational marijuana is legal.

“Maybe these laws aren’t that relevant to them,” she noted.

On the other hand, the new laws open up the doors for adults.

Hasin said the availability and the marketing surrounding marijuana in these states may “normalize the use” of the drug for adults.

Armentano, however, thinks these explanations may be just blowing smoke.

He notes that the first time period in the study was 1991 when marijuana use in the United States was at an all-time low.

He says the increase in subsequent years may be from factors other than marijuana legalization laws.

He also thinks there may be a political agenda here.

“For decades, the primary focus of drug prevention groups and federal agencies was to address concerns with regard to youth access and use,” Armentano told Healthline. “So it is notable that these same agencies are now pivoting to focus on adult use. This shift is likely the result of the fact that their prior claims regarding greater youth consumption and access post-marijuana regulation have not come to fruition.”

Read more: Telemedicine makes it easier to obtain medical marijuana »

So, do medical marijuana laws do more harm than good?

None of the experts seem ready to make that argument, at least not yet.

Dr. Wilson Compton, the deputy director of the National Institute on Drug Abuse, said the recent research was a “strong design to test the implications” of the new marijuana laws.

He said these laws vary significantly from one jurisdiction to another, so he thinks it is worthwhile to study the legalization regulations on a state-by-state basis.

That would be a way to try to determine what portion of the laws could be encouraging illegal marijuana use and what portions aren’t.

Then, he said, officials could weigh the health benefits of medical marijuana to people like cancer patients who need the medication against the risks of increased use among the general population.

“That could have a big impact,” Compton told Healthline. “The impact could be very different in different places due to the implementation of the laws.”