Those who smoked daily or started before age 20—and particularly before age 15—experienced the most health problems, even if they’d stopped smoking.
“Our results show the damage is permanent,” the authors wrote in the study. “What teenagers use on average when they start to use marijuana imposes health costs on them for the rest of their lives.”
Teen smoking also predicted regular use into adulthood. Meanwhile, only .03 percent of people who smoked for the first time at age 21 or over continued to do so daily or almost daily.
Another recent University of Montreal study found that only those who started smoking before age 17 still had cognitive impairments when they were 20.
These findings are in line with the Canadian Task Force on Cannabis Legalization and Regulation’s recommendations to restrict use to adults, according to a press release. Canada plans to legalize weed in 2018, but only for those 18 and over, as we recently reported.
Still, the researchers admit a lot of uncertainty about the longterm effects of marijuana remains, possibly because there aren’t a lot of studies that take place over an extended amount of time. But here’s what experts do know conclusively about the drug.