Teen use of marijuana rose in Washington state after the drug was legalized for adults 21 and older, a new study finds. In Colorado, legalization had no impact on marijuana use by teens, CBS News reports.

The findings come from a study published in JAMA Pediatrics.

In Washington, since 2012, marijuana use among eighth graders has increased by 2 percent and among tenth graders by 4.1 percent. In Colorado there was no increase in marijuana use among teens. The conflicting results suggest a need for more research on the impact of marijuana legalization on teens, said study co-author Magdalena Cerda.

“We know that early initiation of marijuana use–that is initiation at adolescence–is associated with a greater risk for marijuana dependence later on in life,” Cerda said.

The researchers recommended investment in evidence-based adolescent substance use prevention programs in states that may legalize recreational marijuana use.