As recreational marijuana dispensaries prepare to open in Massachusetts, you may be wondering whether it’s safe to indulge. Is pot good or bad for your health?
You’ll find no shortage of anecdotes and opinions in answer to that question.
But if you want the verdict of hard science, you’re pretty much out of luck.
Although marijuana has been studied in many ways over many years, the studies vary in quality and often reach conflicting conclusions, according to experts on the issue.
“We’re in a situation where policy is running ahead of the science,” said Neal Shifman, chief executive of Advocates for Human Potential, a consulting group that organizes national conferences to discuss cannabis policy and science. “There’s a hunger in the marketplace for real information.”
The federal Drug Enforcement Administration classifies marijuana as among the most dangerous drugs, making it very difficult for researchers to obtain the plant for study. Nearly all the research to date involves either purified ingredients from the cannabis plant or smoked marijuana.
But marijuana has dramatically increased in potency in recent years. And today, people can rub marijuana oils on the skin, inhale highly concentrated cannabis vapors, or munch on pot-infused candies and cookies. The effects of the drug consumed in these ways have not been studied.
Amid the uncertainty, discussions tend toward pro-or-con polarization, said Dr. Kevin P. Hill, director of addiction psychiatry at Beth Israel Deaconess Medical Center.
“There are still many, many people who spend a lot of energy trying to make cannabis a simple topic — ‘the greatest medication ever’ or ‘if you use it you’re doomed,’ ” Hill said. “Neither of those is true.”
So what is true?
“If you use cannabis today, we’re pretty sure about what it can do to you,” Hill said, ticking off impairment of judgment, learning, memory, attention, and physical performance, and raising your heart rate. These reactions wear off within a day or so, he said.
But when it comes to long-term effects, the answers get murky.
A couple of years ago, the National Academies of Sciences, Engineering, and Medicine reviewed 10,000 studies of marijuana’s health effects. The result: Conclusive or substantial evidence could be found for only a handful of findings, meaning that several good studies support the finding and few refute it.
These are the beneficial effects that the National Academies concluded are supported by strong evidence:
■ Cannabis helps relieve chronic pain.
■ A cannabis ingredient (cannabidiol) taken orally reduces vomiting from chemotherapy and eases painful muscle spasms in people with multiple sclerosis.
The academies also found strong evidence that these negative effects occur more frequently among people who use marijuana:
■ the development of schizophrenia or other psychoses in adolescents;
■ an increased risk of motor vehicle crashes;
■ lower birth weight when the mother smokes during pregnancy;
■ worse respiratory symptoms and more frequent bronchitis in long-term users.
Many experts also agree that adolescents and adults younger than 25 should avoid marijuana because it can alter their developing brains.
One other effect is known for sure about marijuana, added Susan Weiss, director of the Division of Extramural Research at the National Institute on Drug Abuse: It can be addictive. About 9 percent of regular users develop cannabis use disorder; that goes up to 17 percent among people who start when they’re young.
Marijuana contains hundreds of chemical compounds, the most powerful of which are delta-9-tetrahydrocannabinol, or THC, and cannabidiol, or CBD.
THC produces the psychoactive effects — the marijuana high. CBD has a role in pain control and also moderates the effect of THC. But many strains of marijuana in use today have high concentrations of THC and little CBD to balance it. The long-term effects of this shift are unknown.
Last month, the Food and Drug Administration approved a purified form of CBD as a treatment for two rare forms of severe epilepsy in children, the first time it approved a marijuana ingredient as a therapeutic drug.
(Two synthetic forms of THC, dronabinol and nabilone, are approved for treating nausea and vomiting from chemotherapy.)
Because of the federal restrictions, researchers’ only legal source for study is a Mississippi farm. But the marijuana plants there are not necessarily identical to those that people get at the dispensary or on the street.
Research typically consists of comparing marijuana users with those who abstain. But the most that such studies can show is a correlation with certain outcomes, not a causal connection.
For example, some studies indicate that people who started heavy cannabis use as teens have lower educational and employment achievement and lower income than those who avoided the drug.
That doesn’t necessarily mean that marijuana directly causes those impairments, though it might have a role. The lower performance might also result from other factors in the lives of people who happen to smoke marijuana — and the marijuana smoking may even be a result of those factors rather than their cause.
The better-designed studies control for confounding factors, such as excluding people who use other drugs or have mental health issues. But even those studies still only show elevated risk — not destiny for everyone who smokes pot.
With such obstacles and ambiguities, the research yields a host of suggestive findings and unanswered questions. Among them:
Staci Gruber, director of McLean Hospital’s Marijuana Investigations for Neuroscientific Discovery project, said her work and that of others suggests that heavy marijuana use starting before age 16 may result in difficulty with abstract reasoning, impulsivity, and problem-solving. Imaging tests show the brain structure changes, resulting in “a different pattern of cognitive performance,” she said.
But it appears these effects can be mitigated by stopping or reducing consumption, and the same effects are not seen in people who start later in life or who consume moderate amounts.
And not every pot-smoking teenager is severely damaged. “Most of these kids are doing just fine,” Gruber added.
In older patients who use marijuana for medical reasons, Gruber has observed the opposite — an improvement in cognitive functioning within three months of starting cannabis use. Perhaps people’s minds clear up when freed of pain or anxiety, or when they stop taking opioids or tranquilizers. But cannabis might also directly improve mental functioning in older people.
Studies show a clear link between smoking marijuana while pregnant and lower birth weight. Otherwise, the National Academies deemed it “unclear” whether smoking marijuana affects the pregnancy or the child after birth.
But Weiss, of the National Institute on Drug Abuse, is worried about indications that children whose mothers used cannabis while pregnant have alterations in brain structure and exhibit behavioral problems as they enter school.
Substantial evidence shows that smoking marijuana as a teen increases the risk of psychosis, particularly for those with a family history of schizophrenia or other psychotic illnesses. Otherwise, the studies are inconclusive to the point of being confusing.
People with psychotic disorders who use cannabis seem to do better on learning and memory tasks. But they also may hallucinate more.
Many people with anxiety or mood disorders use cannabis, but any relationship is murky. People may smoke marijuana to treat the symptoms of a mental disorder, or other factors may lead to both marijuana use and mental illness.
Still, some evidence suggests that regular cannabis use might increase the risk for developing an anxiety disorder, and it might also worsen the symptoms of bipolar disorder and increase thoughts of suicide.
Regular smoking is associated with chronic cough and phlegm production. Quitting seems to resolve those problems. The National Academies couldn’t find clear evidence on its effect on other lung diseases.
But Suzaynn Schick, who studies the chemistry and toxicity of smoke at the University of California San Francisco, believes the same health effects seen with tobacco, including the hazards of secondhand smoke, will eventually come to light.
“It doesn’t matter whether you’re burning tobacco, wood, diesel, or marijuana. When you burn things, you create toxins,” she said. “Cannabis can be a medicine, but smoking it is a really stupid way to get your medicine.”Felice J. Freyer can be reached at email@example.com. Follow her on Twitter @felicejfreyer.