A new study published in Lancet Psychiatry Thursday revealed that more Americans are using marijuana on a daily or almost-daily basis than they were 12 years ago. Study authors analyzed data from 596,500 adults aged 18 or older who took part in the annual U.S. National Survey on Drug Use and Health from 2002 to 2014, and found the number of Americans lighting up jumped from 3.9 million to 8.4 million.

That’s because fewer Americans consider cannabis harmful, according to the study. While nearly 51% of the 595,500 adults. respondents said smoking weed once or twice a week had potential to be “greatly harmful” in 2002, only 33.3% perceived the substance that way in 2014.

Total marijuana use increased from 10.4% in 2002 to 13.3% in 2014, too: American adults said they used the drug more often, taking it an average of 16.3 days in a year in comparison to 10 days a year in 2002, the study found.

These findings come as more states legalize marijuana use for medical or recreational purposes. But lead study author Dr. Wilson Compton, of the National Institute on Drug Abuse at the National Institutes of Health, said it’s hard to tell if this push for legalization is the direct cause of increased use over the past decade.

“It is not such an easy question to answer,” Compton said in an interview with Fortune. “States that have medical marijuana laws have higher rates of use … but you don’t know what the direction of causation goes.”

This is to say that increased use could be a result of legalization, but it could also be the result of lifted bans on marijuana, Compton said.

In years past, there have been exaggerated claims about both weed’s harmfulness and harmlessness, according to Compton. It goes to show that the public needs a better understanding of cannabis’ effects, which is hard to do when available research on the drug’s properties are still sparse.

Additionally, while clinicians and physicians will ask their patients about alcohol and tobacco use during visits, medical practitioners should make a point to regularly ask about the frequency of use also be asking more regularly about marijuana use since adult Americans are using the drug even more often than before, Compton said.

A press release for the study included a comment from Wayne Hall, a professor at the University of Queensland in Australia, who thinks it may be too soon to draw any firm conclusions about marijuana and its related harms.

“But,” Hall added, “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.”