Is marijuana really an effective drug? Surprisingly, scientists have no solid answer

One would think that with medical marijuana now legal in 23 states, the science to support its efficacy would be fairly definitive. Surprisingly, that's not the case.

Despite the fierce political tussles and competing medical claims the truth is this: Very little solid scientific evidence exists to either confirm or dispute marijuana’s effectiveness as a drug or its potential for harm.

Marijuana is still classified by the US federal government as a Schedule I controlled substance (meaning it has “no currently acceptable medical use”), so serious research into marijuana is extremely difficult, especially FDA-approved clinical trials that would either prove or disprove marijuana's health benefits. 

States that have approved marijuana for medical use have done so through a legislative process, not a scientific one, researchers say. And that’s a real problem, both for scientists trying to determine marijuana’s effects and for people seeking cannabis to treat chronic medical conditions.

The media isn’t helping. Parents of children with epilepsy may believe, based on what they see in the media, that marijuana is almost a miracle cure, despairs one researcher. But anecdotal evidence is not the same as scientific evidence — and the scientific evidence for marijuana’s efficacy is pretty limited at this point, says Kristen Park, an assistant professor of pediatrics and neurology at the University of Colorado.

Research done in the 1970s and 1980s suggests that the active ingredient, cannabis, has anti-seizure effects, so there is a scientific foundation to the concept, but the data is mixed, according to Park.

“THC, the main component responsible for the high of pot, is mixed in its effects,” she says. “Some of the other cannabinoids, including cannabidiol (CBD), are felt to be more anti-convulsant than THC.”

Data from studies designed for safety and dosage indicate that about 30 percent of children who suffer from seizures respond to marijuana treatment when given in a controlled setting, Park says. But it's important to point out, she notes, that some of the FDA approved epilepsy drugs have a placebo response rate of the same 30 percent.

Complicating matters further, the marijuana available to researchers is totally different from the marijuana dispensed in states where it has been legalized, says Sam Wang, a pediatric emergency medicine physician and medical toxicologist at Children's Hospital of Colorado in Aurora.

“The marijuana in Colorado is very potent — completely different from what a federal research organization like the National Institutes of Health (NIH) or the National Institute on Drug Abuse (NIDA) would provide,” Wang says.

Federally-dispensed NIDA marijuana — the only kind legally available to researchers — has about 6 percent THC in it. The various strains of marijuana available in Colorado contain between 4 and 30 percent THC. The strain used for kids with pediatric epilepsy has forty times less THC and forty times more CBD than typical recreational marijuana.

In reality, there is no longer any one drug that can be labelled marijuana. In places where marijuana has been legalized there are hundreds of different strains, each with its own balance of chemical properties and potential combination of cannabinoids.

And since federally-funded researchers must limit their studies to marijuana provided by NIDA, their research yields data that has virtually nothing to do with what people are actually using.

“We can only be observational about it,” Wang says. “We're not allowed to handle, distribute or dose the marijuana kids are obtaining here. We can watch what they normally do, and then try to get the best available data.”

One of the biggest concerns among doctors and scientists is how marijuana affects the developing brain in the long-term.

“There's solid evidence that suggests marijuana does have deleterious effects in terms of cognition and cognitive processes — short-term memory, attention, things of this nature,” says Kent Hutchison, a professor of psychology and neuroscience at the University of Colorado in Boulder. “We also know that there's an association between adolescent use — especially heavy, frequent use — and negative educational outcomes. So I think everybody agrees that adolescent use of marijuana is not a good thing.”

But the evidence is shakier regarding the long-term effects of marijuana on the adolescent brain, he says. Some studies suggest lingering bad effects; other studies have failed to replicate these results.

A new initiative at NIH, called the Adolescent Brain Cognitive Development Study, aims to provide more clarity.

“The plan is to recruit 10,000 10-year-olds before they've ever used marijuana or alcohol or other drugs, and basically assess their brain and cognitive function over 10 years,” Hutchison explains. “I think that is going to be the gold standard in terms of determining exactly what is the relationship between marijuana, alcohol use and other drugs and changes in the adolescent brain.”

But even after this, the conclusions likely won’t be definitive, Hutchison suggests.

“Even if you find, for example, an association between some change in the brain and marijuana use, you never know which came first,” he says. “Did the marijuana use cause the change in the brain or was there some difference in the brain that made adolescents more at risk for experimenting with marijuana use?”

More research will be necessary and much of it will have to focus on understanding the safety profile of different strains of marijuana.

“In states that are post-legalization, we have to start thinking about marijuana as more than one thing,” Hutchison insists. “We can't compare marijuana in kids for pediatric epilepsy with marijuana used recreationally by adolescents in terms of how it damages the brain.”

“We really need to talk about which strains have potentially the most harmful effects, which strains the least harmful effects, which strains maybe have the most medical benefits, which strains have least medical benefits,” he continues. "In other words, we really need a more nuanced experimental research approach to this. We're still stuck in the 70s and 80s in terms of the laws governing the research, yet states are free to experiment in terms of regulation.”

“There's freedom to regulate, but no freedom to do the research that will inform the regulation. I think that's a problem," Hutchison concludes.

This article is based on an interview that aired on PRI's Science Friday with Ira Flatow