So far, more than half of all U.S. states have legalized marijuana for medical use, and eight (plus the District of Columbia) have legalized the drug for recreational use. Varieties of cannabis available today are more potent than ever and come in many forms, including oils and leaves that can be vaped, and lots of edibles, from brownies and cookies to candies — even cannabis gummy bears.
A report published Thursday by the National Academies of Sciences, Engineering and Medicine analyzed more than 10,000 studies to see what could conclusively be said about the health effects of all this marijuana. And despite the drug's increasing popularity — a recent survey suggests about 22 million American adults have used the drug in the last month — conclusive evidence about its positive and negative medical effects is hard to come by, the researchers say.
According to the report, that's at least partly because the federal drug enforcement agency's designation of the drug as a Schedule I substance — having "no currently accepted medical use and a high potential for abuse" — entails so many restrictions that it has been difficult for researchers to do rigorous research on marijuana.
We just need "far more information," Dr. Marie McCormick, chair of the NAS committee and professor of pediatrics at Harvard Medical School, tells Shots.
Some of the highlights of her committee's 337-page report on marijuana include:
- Pain relief Regarding chronic pain, there's evidence that patients who are treated with cannabis or cannabinoids "are more likely to experience a significant reduction in pain symptoms," the researchers say. More particularly, for adults with muscle spasms related to multiple sclerosis, there is "substantial evidence," they say, that short-term use of certain oral cannabinoids can improve symptoms. And for adults with chemotherapy-induced nausea and vomiting, "there is conclusive evidence" that certain oral cannabinoids are effective in preventing and treating those ailments.
- Cancer There is no evidence that smoking marijuana increases the risk for cancers often associated with tobacco use, such as lung and head and neck cancers, the scientists report, adding that more research is needed to determine whether marijuana use is associated with heart attack and stroke. However, they say, "some evidence suggests smoking marijuana may trigger a heart attack among individuals with diagnosed heart disease." There's also some evidence that smoking marijuana during pregnancy is linked to lower birth weight in the offspring.
- Asthma and other chronic respiratory problems Evidence suggests that smoking marijuana on a regular basis is associated with more frequent chronic bronchitis and worse respiratory symptoms, such as chronic cough, the scientists say. But they add that it's unclear whether the drug increases the risk of developing asthma or chronic obstructive pulmonary disease.
- Mental health problems While the report notes some research suggesting that using marijuana might increase risk of developing schizophrenia or other social anxiety disorders, the committee of scientists cautions that there may be other explanations for that link; it could simply be, for example, that people with these mental health problems are more likely to smoke marijuana.
- Injury or death In terms of risk to life and limb, the NAS committee found that driving under the influence of cannabis "increases the risk of being involved in a motor vehicle accident." The report also cited evidence that increasing numbers of young children may be accidentally ingesting marijuana products now, particularly in states where cannabis use is legal.
- Substance abuse disorders Some parents worry that their teens' use of cannabis can cause problems, and the report was mixed in its conclusions on that. The evidence that marijuana is a gateway drug to trying other drugs, including tobacco, "is limited," the researchers say. But they found "moderate evidence" that there's a link between cannabis use and the development of substance dependence or abuse problems with alcohol, tobacco and illicit drugs. What's more, the evidence suggests that initiating marijuana use at a younger age, the researchers say, "increases the likelihood of developing dependency, which can affect academic performance and social interactions."
"The adolescent brain is very sensitive to these kinds of substances," McCormick says. "So they continue to use it — and may use it in increasing amounts — and are at risk for developing problematic cannabis use."
Erik Altieri, who directs the National Organization for the Reform of Marijuana Laws, says he thinks the legalization of marijuana by many states may actually reduce the problematic use of the drug by teens.
"That's because we are taking marijuana off of the street corner," he says, "and out of the hands of drug dealers, who have nothing but incentive to sell to everyone and anyone." Legalizing the drug, he points out, puts it "behind the counter of a regulated business that has to check for ID, answer to the government and has oversight."
So far, states that have legalized recreational marijuana have not seen an increase in use among underage teens, Altieri says.
"By legalizing it and normalizing it," he says, "it's become just another everyday thing that adults partake in — it doesn't have that same draw to it that it used to."
Still, McCormick says, many health questions remain to be answered by better research. The increased legal availability of cannabis products in many states, and their increased potency, she says, make that rigorous research more important than ever.
ROBERT SIEGEL, HOST:
More than half of all the states have legalized marijuana, also known as cannabis, for medical use, and eight states have legalized it for recreational use. Well, now a report from the National Academies of Sciences, Engineering and Medicine examines what we know about the health effects of marijuana. NPR's Patti Neighmond reports.
PATTI NEIGHMOND, BYLINE: Researchers looked at more than 10,000 scientific studies. Marie McCormick with Harvard's T.H. Chan School of Public Health headed the research which is timely not only because marijuana is more widely available and used. It's also stronger than ever and comes in lots of new and different forms.
MARIE MCCORMICK: There's vaping, which is same as it is for nicotine. There's dabbing, which is taking a concentrated form, heating it up and inhaling the vapor. There is also edibles - the classic marijuana brownie but also chewy bears and things of that sort.
NEIGHMOND: The report explores 11 different health conditions. It finds cannabis can be beneficial when it comes to chronic pain or nausea related to chemotherapy.
MCCORMICK: Some people with chronic pain, muscle spasms for multiple sclerosis or nausea and vomiting from cancer chemotherapy obtained some relief of their symptoms from using cannabis-based products or cannabis.
NEIGHMOND: But there are potential harms. Marijuana may increase the risk of developing schizophrenia, although McCormick says it could be that people with the disorder are more likely to smoke it. The drug may also increase the risk for certain social anxiety disorders. For pregnant women, marijuana, like cigarettes, can increase the risk of having a low-birth-weight baby.
MCCORMICK: It's generally thought that smoking cannabis limits the growth of the infant, limits the effectiveness of the transfer of nutrients across the placenta.
NEIGHMOND: Smoking marijuana regularly is associated with more bouts of bronchitis and other respiratory problems. People already diagnosed with heart disease may have an increased risk of heart attack.
But for healthy individuals, there seems to be no increased risk of stroke or cancer, including tobacco-related lung and head cancers. And while marijuana does not seem to increase the use of other drugs or tobacco products, it may increase the risk of dependency, particularly among younger users.
MCCORMICK: The adolescent brain is very sensitive to these kinds of substances. And so they continue to use it and may use it in increasing amounts and are at risk for developing problematic cannabis use.
NEIGHMOND: Which can impair functioning, both academically and socially. Erik Altieri directs the National Organization for the Reform of Marijuana Laws, or NORML, which wants to see marijuana legalized for adult use nationwide. He says lots of research finds little harm in marijuana use. And a hidden benefit of legalization - he says it could actually reduce marijuana use among teenagers under 18.
ERIK ALTIERI: That's because we are taking marijuana off of the street corner, out of the hands of drug dealers who have nothing but incentive to sell to everyone and anyone, putting it behind the counter of a regulated business that has to check for ID, that has to answer to the government and has oversight.
NEIGHMOND: And in states that have legalized recreational marijuana, Altieri says there has not been an increase in use among underage teens in part because it may be harder to get and in part because the cool factor is lessened.
ALTIERI: By legalizing it and normalizing it, it's become just another everyday thing that adults partake in. It doesn't have that same draw to it that it used to.
NEIGHMOND: Even so, researchers say a lot more information about potential harms of marijuana needs to be studied and understood. Patty Neighmond, NPR News.
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