In 2017, some Americans could buy legal marijuana almost as easily as they could order a pizza. Pot shops outnumber Starbucks stores in states like Colorado and Oregon, and medical marijuana delivery services drop the drug off at people's doors in California and Massachusetts.
With marijuana now more accessible — legally — than ever, more researchers are weighing in on its health effects. But that doesn't mean we fully understand the plant or its impacts.
The National Academies of Sciences, Engineering, and Medicine released a massive report in January that gives one of the most comprehensive looks — and certainly the most up-to-date — at exactly what we know about the science of cannabis. The committee behind the report, representing top universities around the country, considered more than 10,000 studies for its analysis and drew nearly 100 conclusions. Many of those findings are summarized below.
Within a few minutes of inhaling marijuana, your heart rate can increase by between 20 and 50 beats a minute. This can last anywhere from 20 minutes to three hours, according to the National Institute on Drug Abuse.
The report from the National Academies found insufficient evidence to support or refute the idea that cannabis might increase the overall risk of a heart attack. The same report, however, also found some limited evidence that smoking could be a trigger for a heart attack.
In August, a study published in the European Journal of Preventive Cardiology appeared to suggest that marijuana smokers face a threefold higher risk of dying from high blood pressure than people who have never smoked. But the study came with an important caveat: it defined a "marijuana user" as anyone who'd ever tried the drug.
Research suggests this is a poor assumption — and one that could have interfered with the study's results. According to a recent survey, about 52% of Americans have tried cannabis at some point, yet only 14% used the drug at least once a month.
Other studies have come to the opposite conclusion. According to the Mayo Clinic, using cannabis could result in decreased — not increased — blood pressure.
So while there's probably a link between smoking marijuana and high blood pressure, there's not enough research yet to say that one leads to the other.
People who smoke marijuana regularly are more likely to experience chronic bronchitis, according to the report from the National Academies. There's also evidence that stopping smoking relieves these symptoms.
Perhaps surprisingly, the report's authors found moderate evidence that cannabis was not connected to an increased risk of the lung cancers or head and neck cancers associated with smoking cigarettes.
Marijuana contains cannabidiol, or CBD, a chemical that is not responsible for getting you high but is thought to be responsible for many of marijuana's therapeutic effects. Those benefits can include pain relief or potential treatment for certain kinds of childhood epilepsy.
The report from the National Academies also found conclusive or substantial evidence — the most definitive levels — that cannabis can be an effective treatment for chronic pain, which could have to do with both CBD and THC, the psychoactive ingredient in marijuana. Pain is "by far the most common" reason people request medical marijuana, according to the report.
In September, a study published in the Journal of American Medical Directors Associationseemed to suggest that marijuana might actually prevent people from graduating to "hard" drugs. The five-year-long study involved 125 participants, all of whom endured chronic pain.
Researchers found that one-third of participants who used marijuana to treat their chronic pain stopped taking prescribed medications. They suggested that legal access to cannabis might lower the use of dangerous prescription medications in certain patient populations.
Drug overdoses are the leading cause of death for Americans under 50.
Sonya Yruel/Drug Policy Alliance
In August, a study published in the journal Neuropsychopharmacology suggested that pot smokers could have a greatly reduced risk of stroke. Researchers from the University of Texas at Dallas found chronic cannabis users have higher blood flow to the brain and extract more oxygen from cerebral blood flow than nonusers. THC is known to relax blood vessels.
These changes effectively reduce the risk of stroke and blood clots.
The study should be taken with a grain of salt, however, as it included fewer than 200 participants.
A drug called Epidiolex, which contains CBD, may be on its way to becoming the first of its kindto win approval from the Food and Drug Administration for the treatment of childhood epilepsy. The company that makes it, GW Pharma, is exploring CBD for its potential use in people with Dravet syndrome, a rare, lifelong form of epilepsy that begins in infancy.
In March, the company came out with phase-three trial data that showed the drug Epidiolex had some positive results. The hope is that eventually, someone taking the drug would be able to either reduce the number of seizures they have or even stop them entirely.
Marijuana can mess with your memory by changing the way your brain processes information, but scientists still aren't sure exactly how this happens. Still, several studies suggest that weed interferes with short-term memory, and researchers tend to see more of these effects in inexperienced or infrequent users than in heavy, frequent users. Unsurprisingly, these effects are most evident in the acute sense — immediately after use, when people are high.
According to the new report from the National Academies, there was limited evidence showing a connection between cannabis use and impaired academic achievement, something that has been shown to be especially true for people who begin smoking regularly during adolescence. (That has also been shown to increase the risk for problematic use.)
Importantly, in most cases, saying cannabis is connected to an increased risk doesn't mean marijuana use caused that risk.
Scientists can't say for sure whether marijuana causes depression, or depressed people are simply more likely to smoke. But a 2011 study from the Netherlands suggested that smoking marijuana could raise the risk of depression for young people who already have a special serotonin gene that could make them more vulnerable to depression.
The new report from the National Academies bolstered those findings. It found moderate evidence that cannabis use was linked to a small increased risk of depression.
AP Photo/Esteban Felix
The new report from the National Academies also found substantial evidence of an increased risk among frequent marijuana users of developing schizophrenia — something that studies have shown is a particular concern for people at risk for schizophrenia in the first place.
In October, a study published in the Journal of Sexual Medicine showed that people who regularly use marijuana have about 20% more sex than abstainers. Researchers from Stanford University based their analysis on a survey of more than 50,000 Americans ages 25 to 45.
Importantly, saying cannabis is correlated to an "increased coital frequency" doesn't mean marijuana use caused more sexual activity. But the results hint at a casual connection.
According to the report from the National Academies, substantial evidence shows a link between prenatal cannabis exposure — when a pregnant woman uses marijuana — and lower birth weight. There was limited evidence suggesting that using marijuana during pregnancy could cause complications and increase the risk that a baby would have to spend time in a neonatal intensive care unit.
AP Photo/Richard Vogel
Based on the available evidence and conversations with researchers, there are good reasons to think marijuana has potentially valuable medical uses. At the same time, we know that not all use is risk-free (as with any substance).
More research is needed to figure out how to best treat the conditions that cannabis can help and how to minimize any risks associated with medical or recreational use.