The Cannifornian is continuing an occasional series that surveys current research and interviews experts on common questions about the health impacts of marijuana use. Have a question you’d like us to consider? Email

Q. Does marijuana cause brain damage?

A. More studies are needed to answer this complex question. But while a growing body of research indicates even heavy marijuana use seems to have few long-term health effects for adults, it may negatively affect developing teen brains.

People who started using cannabis as teenagers lost about eight IQ points by the time they were 38, according to a 2012 study by Madeline Meier, a psychologist at Arizona State University.

Overwhelmingly, studies by Staci Gruber, a psychiatry professor at Harvard Medical School and director of the Marijuana Investigations for Neuroscientific Discovery team at Boston’s McLean Hospital, show that chronic marijuana smokers who start using before age 16 perform more poorly on cognitive tests. They show reduced impulse control and issues with staying focused on a task, she said.

Even the brain structure itself can be altered in young smokers, she said, decreasing the amount of white matter, which affects how we learn.

“It’s not as if these people appear to have brain injuries,” she said. “In all actuality, these people are able to function the same as people who don’t smoke.”

Other studies contradict such results, and some researchers question whether family environment and other factors account for some of the cognitive decline. And even if IQ declines are correlated to marijuana use, it’s tough to know whether the drug changed the brain or subjects simply weren’t paying attention in school because they were high.

“I think it’s fair to say, as far as heavy adolescent use, we have an open question here,” said Dr. Igor Grant, a psychiatrist who oversees the Center for Medical Cannabis Research at UC San Diego.

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Most researchers agree such unanswered questions justify regulations – akin to those included in California’s Proposition 64 – that are aimed at keeping marijuana away from young people. So, where should the age line on legal use be drawn?

In her study, Meier said negative impacts on IQ disappeared for people who started using at age 18, which is the age limit for California medical marijuana patients.

However, all eight states that have legalized recreational use set 21 as the age limit.

Gruber said brain development continues until around age 25, making it tough to say that marijuana users are “out of the woods” at 21.

Q. Can you overdose on marijuana?

A. Unlike with alcohol and most other drugs, researchers say there’s been no recorded death attributed to using too much cannabis. But experts say overdoing it can cause highly unpleasant and, rarely, dangerous experiences.

Consuming large amounts of potent pot can temporarily trigger rapid heartbeats, nausea and hallucinations.

“It’s not that you’ll die from it,” Gruber said. “But you might be so sick that you’ll wish you could die.”

“Bad trips” typically happen when inexperienced consumers try a cannabis-infused food, don’t feel anything after 30 minutes and eat more. Since the effects of cannabis edibles can take up to two hours to be felt, it’s easy to overindulge.

Most adverse symptoms wear off within a couple of hours, so medical experts suggest users relax and have a sober friend watch over them. They stress driving while impaired can be dangerous, as can the actions of someone who slips into a severe state of psychosis.

A lawsuit pending in Denver against an edibles company claims a man ate large quantities of cannabis-infused candy, started hallucinating, then killed his wife. And in the last two years, officials or family members have attributed two Colorado suicides to high doses of cannabis edibles.

Colorado now requires edibles to be sold in serving sizes of 10 milligrams, which it considers one dose. There’s also a push for stricter packaging and labeling requirements to help consumers dose more wisely.