Parents worrying that the legalization of marijuana for medicinal purposes will allow their teenage children greater access to the drug can breathe easy. According to a new study in the Journal of Adolescent Health, the legalization of medical marijuana has no correlation with increased teenage use.

The research is based on a study of 20 years of data taken from US states with and without medical marijuana laws.

California became the first US state to legalize medical marijuana in 1996. Since then 20 other states and the District of Colombia have followed suit.

"Any time a state considers legalizing medical marijuana, there are concerns from the public about an increase in drug use among teens," said Dr. Esther Choo, the study's principal investigator and emergency medicine physician at Rhode Island Hospital.

"In this study, we examined 20 years worth of data, comparing trends in self-reported adolescent marijuana use between states with medical marijuana laws and neighboring states without the laws, and found no increase in marijuana use that could be attributed to the law," Choo said in a statement.

Choo said the research adds to a growing body of evidence that consistently indicates that state medical marijuana policies "do not have a downstream effect on adolescent drug use," which has long been a concern. 

Not all research comes to the same conclusion, however. A more general study from earlier this year suggested that teen marijuana use will likely increase as marijuana becomes more legally available, citing a number of teens who said they have not tried the drug becuase it is currently illegal, but would likely try it if it were legal. 

For their study, Choo and her collaborators analyzed data from a nationally representative sample of high school students. Among those surveyed, marijuana use was reported by about 21 percent, indicating that using the drug was fairly common.

However, Choo reports that there were no statistically significant differences in the students' marijuana use before and after state policies legalizing the drug for medical use were enacted, regardless of the state pairings analyzed.

"Researchers should continue to monitor and measure marijuana use," Choo said. "But we hope that this information will provide some level of reassurance to policymakers, physicians, and parents about medical marijuana laws."

Another recent study on medical marijuana found no correlation in increased crime rates with legalization of medical marijuana.