If You Are Considering Smoking Marijuana—or Quitting—Here Is What You Need to Know

Health and medicine explained.
May 1 2014 8:26 AM

How Bad Is Marijuana for Your Health?

What recent press coverage gets wrong.

Participants smoke marijuana around 4:20 pm at the Seattle Hempfest 4/20.
Participants smoke marijuana around 4:20 p.m. at the Seattle Hempfest 4/20 event on April 20, 2014. Some members of the media might call this casual marijuana use.

Photo by Jason Redmond/Reuters

The Journal of Neuroscience recently published a study linking recreational marijuana use to subtle changes in brain structure. The researchers, led by Jodi Gilman of Massachusetts General Hospital, identified increased gray matter density in the left nucleus accumbens and some bordering areas. The study was fine, but the media coverage was abysmal. Reporters overstated the findings, mischaracterized the study, and failed to mention previous research done on pot smoking and health. Goldfish may not have a three-second memory, but some journalists seem to. When a new paper comes out, it’s often treated as the first ever and final word on the topic. There is a significant body of literature on the neurological and wider health effects of marijuana, and to ignore it when covering new studies seems to me a form of journalistic malpractice.

A press release from the Society for Neuroscience trumpeted the Gilman study’s importance because it looked at casual users rather than regular pot smokers, who form the basis of most marijuana studies. That claim is dubious in the extreme. The subjects averaged 3.83 days of smoking and 11.2 total joints per week. Characterizing these people as casual pot smokers was a great media hook, but it defied common sense. Occasional users wondered if they’d done permanent damage, and parents were concerned that their teenagers might face profound neurological changes from experimenting with pot. Any reporter who read the study, however, should have known not to take that bait.

Even by the standards of past medical studies, it’s a stretch to call these subjects casual pot smokers. Just two years ago, for example, Janna Cousijn and colleagues published a study on a group that she called “heavy” marijuana users. In the average week, they smoked 3 grams of cannabis—approximately 2 grams less than Gilman’s casual smokers. (A joint has about 0.5 grams of cannabis.) The justification for calling Gilman’s subjects casual smokers is that they didn’t meet the criteria for dependence, but when you count up the joints, the study doesn’t look so revolutionary.

Advertisement

Many stories also claimed that the Gilman study showed direct causation between pot smoking and brain abnormalities. That’s wrong. The study looked at differences between pot smokers and abstainers at a single moment. Only a longitudinal study, examining brain changes over time, could have suggested causation. As a letter writer to the Journal of Neuroscience noted, it’s possible that pre-existing brain differences cause some people to seek out marijuana. Gilman’s pot smokers also drank more and smoked more cigarettes than the control group, which supports this interpretation and also raises the possibility that other factors led to brain structure differences.

The biggest problem with the coverage of the marijuana study was that it failed to put the new research into context. Valentina Lorenzetti of the University of Melbourne recently published a widely cited review paper synthesizing dozens of studies on marijuana and the brain. Taking the literature as a whole, there is evidence suggesting that marijuana use causes structural changes in three parts of the brain: the frontal lobes, temporal lobes, and the cerebellum. The data also reinforces the idea that long-term, heavy smokers experience greater changes than casual users. The studies, however, have serious flaws. They are typically small and have been unable to show that the structural changes cause cognitive impairment. Gilman’s study of 20 smokers is a good contribution to the literature, but it doesn’t resolve those problems.

TODAY IN SLATE

Jurisprudence

Don’t Expect Adrian Peterson to Go to Prison

In much of America, beating your children is perfectly legal. 

Ken Burns on Why Teddy Roosevelt Would Never Get Elected in 2014

Cops Briefly Detain Django Unchained Actress Because They Thought She Was a Prostitute

Minimalist Cocktail Posters Make Mixing Drinks a Cinch

How the Apple Watch Will Annoy Us

A glowing screen attached to someone else’s wrist is shinier than all but the blingiest of jewels.

Books

Rainbow Parties and Sex Bracelets

Where teenage sex rumors come from—and why they’re bad for parents and kids.

Books

You Had to Be There

What we can learn from things that used to be funny.

Legendary Critic Greil Marcus Measures and Maps Rock History Through 10 Unlikely Songs

Catfish Creator Nev Schulman’s Book Is Just Like Him: Self-Deluded and Completely Infectious

Behold
Sept. 12 2014 5:54 PM An Up-Close Look at the U.S.–Mexico Border
  News & Politics
Jurisprudence
Sept. 14 2014 2:37 PM When Abuse Is Not Abuse Don’t expect Adrian Peterson to go to prison. In much of America, beating your kids is perfectly legal. 
  Business
Moneybox
Sept. 12 2014 5:54 PM Olive Garden Has Been Committing a Culinary Crime Against Humanity
  Life
Inside Higher Ed
Sept. 13 2014 8:38 AM “You’re More Than Just a Number” Goucher College goes transcript-free in admissions.
  Double X
The XX Factor
Sept. 12 2014 4:05 PM Life as an NFL Wife: “He's the Star. Keep Him Happy.”
  Slate Plus
Behind the Scenes
Sept. 12 2014 5:55 PM “Do You Know What Porn Is?” Conversations with Dahlia Lithwick’s 11-year-old son.
  Arts
Brow Beat
Sept. 14 2014 7:10 PM Watch Michael Winslow Perform Every Part of “Whole Lotta Love” With Just His Voice
  Technology
Future Tense
Sept. 12 2014 3:53 PM We Need to Pass Legislation on Artificial Intelligence Early and Often
  Health & Science
New Scientist
Sept. 14 2014 8:38 AM Scientific Misconduct Should Be a Crime It’s as bad as fraud or theft, only potentially more dangerous.
  Sports
Sports Nut
Sept. 12 2014 4:36 PM “There’s No Tolerance for That” Pete Carroll and Jim Harbaugh say they don’t abide domestic abuse. So why do the Seahawks and 49ers have a combined six players accused of violence against women?