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.
(SOUNDBITE OF GENERATIONALS SONG, "SAY WHEN")