Medical marijuana advocates now have evidence that legalizing pot for medical purposes does not lead to increased underage usage of the drug. A massive study analyzing data on 1 million teenagers in 48 states found legalizing medical marijuana actually reduced teen usage, the Guardian reported.

The study, published in the medical journal Lancet Psychiatry, tracked 1,098,270 8th-grade, 10th-grade and 12th-grade students over 24 years. The teens were asked if they had smoked or used marijuana products in the prior month. Teenage pot use didn’t increase with the rise of legal medical marijuana in 21 states as of 2014, the study found. Instead, teenage usage decreased from 8 percent before legalization to 6 percent afterward.

With Alaska and Hawaii, a total of 23 states have legalized medical marijuana. Maryland, Minnesota and New York legalized pot for medicinal purposes last year, according to the website ProCon.org. Last week, Delaware lawmakers decriminalized possession of less than an ounce of pot.

In states that passed medical marijuana laws after the study began, teen marijuana use was already slightly higher than in other states. Usage was about 16 percent in those states, compared to 13 percent in states that still lack medical marijuana laws.

The difference could be due to the more liberal attitudes toward the drug in medical marijuana states, LiveScience reported. "Our study findings suggest that the debate over the role of medical marijuana laws in adolescent marijuana use should cease, and that resources should be applied to identifying the factors that do affect risk," researchers wrote.

The researchers also said pot use decreased among eighth graders after medical marijuana was legalized because the teens started to see pot as a relatively harmless medical product. That "certainly doesn't fit with the idea of being a rebellious teenager," marijuana industry expert Debra Borchardt wrote for Forbes.