Medical marijuana laws do not trigger an increase in teen pot smoking, according to a study published Monday.

Some opponents of medical marijuana have said that legalizing the medicinal use of the drug could send a message to young people that smoking pot is no big deal, ultimately encouraging them to experiment with marijuana and harder drugs.

Pot smoking by teens has been increasing, and earlier research has shown that fewer of them see marijuana as risky. But the new study published in The Lancet medical journal suggests that medical marijuana laws are not the reason.

The research showed no significant increase in 21 states with medical marijuana laws.

“Our findings provide the strongest evidence to date that marijuana use by teenagers does not increase after a state legalizes medical marijuana,” lead author Deborah Hasin, a researcher at Columbia University in New York, said in a statement.

The study is based on an ongoing government-funded survey of 8th, 10th and 12th graders, which asks about marijuana use in the previous month. The researchers reviewed responses from more than 1 million students in 48 states, from 1991 through 2014.

Using that data, the researchers sought to answer two questions: whether marijuana use was higher in states that passed medical marijuana laws and whether the risk of marijuana use changed after passage of medical marijuana laws.

The researchers found that marijuana use tended to already be higher in states that went on to adopt medical marijuana laws. But they did not see an additional spike after the laws were passed.

In fact, the researchers saw a decline in marijuana use by 8th graders in those states. The laws may have caused 8th graders to be less likely to think of pot as a recreational drug, the researchers speculated. Or it's possible the new laws resulted in some parents working harder to stop kids from trying it, they added.

“This study draws attention to the importance of undertaking rigorous scientific research to test hypotheses and using the results to develop sensible health policies,” wrote Dr. Kevin Hill, a substance abuse expert at McLean Hospital in suburban Boston, in an editorial accompanying Hasin's article.

Al Jazeera and The Associated Press