Harris instead talks about cannabinoid receptors and brain development, about how regular marijuana use may affect performance on a test now, or 10 years from now. As states loosen their laws around limited marijuana use, raising concerns that it could cause an increase in use by teenagers, recent studies have found that marijuana dependency among teens can change their brains for the long term.

Voters in November will consider a ballot question that would make Massachusetts the 18th state to allow medical use of marijuana. Advocates of such laws say marijuana offers patients with chronic conditions like cancer and Crohn’s disease relief from pain and nausea, and research has begun to show such benefits. But experts in drug policy and public health say they worry the change could shift teens’ perspective, making them see the drug as curative and safe for recreational use.

“I think we’re going to be sacrificing the mental health of our young people if we pass this law,” said Dr. John Knight, director of the Boston Children’s Hospital Center for Adolescent Substance Abuse Research, where Harris also works.

Recent studies have found that marijuana use may cause or worsen mental health problems in long-term and regular users. Two 2010 reviews of the medical literature related to schizophrenia and psychosis said the research suggests marijuana may bring on the disorders or worsen symptoms, particularly in young people already genetically predisposed to the conditions. Both reviews noted gaps in the science and said more study is needed.

A study published last month that linked regular marijuana use to a decline in IQ was more definitive. A team of researchers, led by Madeline Meier of Duke University’s Center for Child and Family Policy, analyzed a New Zealand study that followed 1,037 people from birth to age 38, periodically assessing marijuana use and neuropsychological functioning.

The study, published by the Proceedings of the National Academies of Sciences, found that those who were diagnosed with marijuana dependency as teenagers and who continued using it regularly into adulthood experienced cognitive decline. The most persistent users who started as teens saw a drop of about 8 IQ points.

“Increasing efforts should be directed toward delaying the onset of cannabis use by young people,” the authors wrote.

They noted that those who started using marijuana as adults did not experience a drop in IQ. That’s because teenage brains are different, Harris said.

Researchers once thought brain development ended in early adolescence. In recent years, though, scientists have found it continues into early adulthood. Marijuana acts on the cannabinoid receptors, thought to be a kind of regulatory system for the brain that plays a role in memory and learning, among other things. Regular marijuana use in adolescence tinkers with those controls when the brain is vulnerable, Harris said.

Those concerned about what the latest findings mean for teens are up against a difficult trend, one Harris attributed in part to the passage of laws legalizing medical marijuana, starting with California in 1996.

In the late ’80s and early ’90s, about 77 percent of high school seniors interviewed in a long-running survey on drug use, called Monitoring the Future, said that people who smoke marijuana regularly are at great risk of harming themselves, physically or otherwise. In 2011, that figure had dropped to 46 percent, close to what it was in 1975.

Knight, of Children’s Hospital, said he lays blame for the shift in attitude among teens “at the public doormat,” for the debate over marijuana as medicine.

Young people “understand that prescribed medicine is safe and effective,” he said. “It has to be proven safe and effective by the Food and Drug Administration. They forget the phrase, ‘when used as directed,’ and so we have a real problem now with prescription painkillers.”

Calling marijuana medicine implies that it is also “safe and effective,” though the drug has not been approved by federal regulators, he said.