"Marijuana is safer than alcohol or tobacco."
I came across that claim again last week while scrolling through Twitter. This time, the tweet was from Dr. Peter Grinspoon, who practices primary care at Mass General.
Grinspoon supports ballot Question 4 in Massachusetts, which would make marijuana legal for recreational use. He has a lot of personal experience with drugs; he's in recovery from an addiction to Vicodin and a few other opioid painkillers. But when asked about his tweet, Grinspoon talks about his patients. He says as many as half of his patients use marijuana, many to relieve pain or in other ways Grinspoon says are a health benefit.
In contrast, Grinspoon says: "You just see life after life swallowed up by alcohol. You just see alcohol tearing families apart. And you don’t have to walk very far to see lives destroyed by tobacco. I’ve never in 20 years as a primary care doctor seen a single life taken away by marijuana."
OK, that’s the opinion of one doctor. Let’s look at the science behind the "safer than" claim in three areas: addiction or dependence, disease, and death.
First: addiction. Here are the most widely accepted rates: 9 percent of marijuana users will become dependent. For alcohol, it's 10 to 15 percent, and for tobacco it's 24 percent. The risk of addiction to all substances appears to be higher if you start as a teenager and use often.
With marijuana, some doctors worry that the percentage of users who become dependent may rise as growers develop strains with higher concentrations of THC.
"As potency increases, addiction cases tend to go up," said Dr. John Kelly, an assistant professor of psychiatry and addiction medicine at Harvard Medical School.
One more point on addiction: Most patients say withdrawal from marijuana is much less intense than from alcohol or tobacco.
Now let's compare illness or disease. We’ve looked at marijuana and the brain in some earlier stories. Here, we’ll talk about lungs. Some researchers say there are reasons to think marijuana might lead to lung cancer.
"Not only because of the carcinogenic chemicals contained in marijuana smoke, but also marijuana smokers tend to inhale more deeply, they tend to have longer breath holding times, and people tend to smoke marijuana cigarettes or joints without filters," said Russ Callaghan, an associate professor of psychiatry at the Northern Medical Program of the University of Northern British Columbia.
When Callahan reviewed 40 years of data on men in Sweden, he found that heavy cannabis smokers are more than twice as likely to develop lung cancer as compared to non-smokers. But Callaghan says many studies show a 15-fold greater chance of lung cancer among men who smoke cigarettes.
"That risk is much higher, so I think what we’re trying to figure out is why the risk is much more reduced [with marijuana] than what we’d expect with tobacco," Callaghan said.
He suggests cannabis may be less likely to lead to lung cancer because the volume inhaled is much smaller — there are very few pack-a-day marijuana smokers.
Callaghan says there are problems with all seven or so long-term studies of cannabis and lung cancer. It's difficult to distinguish the effects of tobacco and marijuana because most heavy cannabis users also smoke tobacco. And it's hard to compare findings because there’s no such thing as a standard size marijuana joint. Callaghan says the research on cannabis and the lungs is three or four decades behind that of tobacco, which he says is a reason to be cautious about increasing access.
But when it comes to death, the really bad guy appears to be alcohol.
Researchers at the Technical University of Dresden in Germany, in a study published last year, estimated the risk of dying after long-term use of 10 recreational substances.
"Alcohol was at the highest risk and cannabis at the lowest risk end," said lead author Dirk Lachenmeier. With alcohol, Lachenmeier looked at the danger of death from cirrhosis across Europe. There was no data clearly linking marijuana to death in humans. Nicotine found in cigarettes posed a high risk of death, but not at high as alcohol.
One reason alcohol is more than 100 times riskier than marijuana, as the study shows: it's everywhere.
"Here in Europe it’s available at every food store, supermarket, petrol stations, and so of course alcohol is much more easily available than the illegal drugs," Lachenmeier said.
Lachenmeier stresses that his findings do not mean marijuana is safe. Some doctors argue asking whether marijuana is less dangerous than tobacco or alcohol is misguided. Dr. Sharon Levy, who directs the adolescent substance abuse program at Boston Children’s Hospital, says people who use marijuana are likely to drink and smoke cigarettes as well.
"So it’s not a really productive health strategy to say, 'Look, if we could get everyone to start smoking marijuana then we would avoid these other substances.' That’s just not something that’s true," Levy said.
More residents of Colorado and Washington report using marijuana since it became legal in those states, although alcohol is still much more widely consumed. Is this casual or heavy marijuana use? And are people swapping one substance for another, or increasing their overall intake of alcohol, tobacco, weed and other drugs? It will be years before we have a clear picture of the health effects, good and bad, of making marijuana legal in states that take that step.
This segment aired on November 2, 2016.