According to a new study published in the International Journal of Drug Policy, legalization of medical marijuana in various US states has resulted in a notable rise in the potency of cannabis – specifically, higher percentages of THC. 

Researchers analyzed 39,157 samples of pot seized by law enforcement in 51 different US jurisdictions over a 20-year span (from 1990 to 2010), and found that potency increased slightly in jurisdictions that legalized medical pot.

However, when specifically examining pot from jurisdictions with dispensaries for medical marijuana, researchers found the potency increased a full percentage point on average.

Rather than attributing this increase to regulations allowing for medical pot cultivation, the study found that higher potency pot is simply more in demand in areas with storefront dispensaries. In other words, the supply of more potent pot is a reaction to the demand for more potent pot.

One of the authors of the study, Eric Sevigny, suggests that in legal recreational states like Colorado, the black market will have to start providing more potent pot if it hopes to compete with retail outlets, despite offering a cheaper, tax-free product.

The study is part of a multi-year endeavor by the RAND Corporation to investigate the effects of legalization.