In 2013, reports that a chemical called cannabidiol (CBD) had reduced the seizures of a 6-year-old girl from near-death levels to almost zero sent desperate patients everywhere on a frenzied quest for treatment.

According to several media outlets, Charlotte Figi was suffering 300 grand mal seizures per week and had lost the ability to walk, talk, and eat. Existing epilepsy medication had failed her. But CBD—a component of cannabis that does not trigger the plant's characteristic high—reduced her episodes to a few per month, and, as her parents told reporters, ushered in a full cognitive recovery.

By most estimates, existing seizure medications fail about one-third of all sufferers, either because the drugs don't stop the seizures or because the side effects are too severe. As Figi's story spread, families with loved ones suffering from this type of epilepsy (which can range from seriously debilitating to life-threatening) began relocating from states where CBD could not be legally obtained to states where it could. At least some of them reported similarly miraculous responses to it. 

So when we reported earlier this month that a new National Academy of Sciences report—the most in-depth analysis of marijuana research to date—concluded that there was not enough evidence to say that cannabis oil could actually treat epilepsy, some people were surprised:

How could the panel draw such a conclusion? Isn’t Figi’s case, and others like it, proof enough that cannabis oil can succeed for epilepsy patients where so many other treatments have failed?

Because readers asked, and because this particular conclusion (one of almost 100 that the academy drew from their scientific review) is a good window into the whole report and the larger debate surrounding medical marijuana, we decided to take a closer look.

Evidence the Panel Considered

The reviewers restricted their evaluation to studies that involved humans, excluding any evidence from studies done on animals. That decision was based on sound scientific rationale (animal, or “pre-clinical,” studies can give us important clues about how a chemical might behave, not whether it will be safe or effective for humans). But limiting the studies in this way left the reviewers with some pretty slim pickings.

There were just three clinical studies where doctors treated patients with CBD and measured whether and by how much those patients’ seizures were reduced. The largest of those studies included a total of 162 patients, treated with 99 percent CBD oil extract for 12 weeks; it found that CBD worked about as well as existing anti-epilepsy medications do in treatment-resistant sufferers.

CBD reduced seizures by a monthly average of 36.5 percent; only five patients saw their motor seizures completely disappear during the study period, and only two patients became completely seizure-free.

Why That Evidence Was Deemed Insufficient

For three reasons, mainly.

First, there was simply not enough of that kind of evidence. FDA-approved drugs are usually deemed effective or ineffective based on large-scale clinical trials that study hundreds to thousands of patients over several years. These studies followed only a few hundred patients in all, and for only a few months. 

Second, the studies in question did not include placebo controls. That is, all of the study participants were given actual CBD, and they knew they were getting it. Other research has found that the placebo effect can be especially strong when it comes to marijuana. In one study, patients of families who moved to Colorado for cannabis oil treatment were twice as likely to report a substantial reduction in seizures as those patients who already lived in the state.

The third major reason the studies were deemed insufficient involves drug interactions. Most of the patients in the studies we’re talking about were taking other anti-epilepsy drugs, such as clobazam and valproate, and it turns out that CBD is very good at blocking the liver enzymes that normally break down those other drugs. This drug interaction makes it impossible to say whether reduced seizure incidence seen in study participants was due to CBD by itself or whether it was simply the result of those other medications staying in the system for longer stretches.

What Seizure Sufferers Should Do

The good news is that higher-quality randomized clinical trials that examine the effectiveness of CBD oil for epilepsy are underway (some have even been completed), and the results should be published sometime this year.

In the meantime, there are a few things to keep in mind if you or your child suffers from intractable seizures and you're considering trying cannabis oil.

First is that, as we've written before, medical marijuana (or cannabis) products are not subject to the same regulations as FDA-approved drugs. Even items sold through a dispensary or a mail-order service may be mislabled or contaminated. 

Second, as noted above, even with design flaws that might have made CBD appear more effective than it really is, the available studies found that for most patients the drug did not work better than existing anti-epilepsy medication in treatment-resistant patients. That is, it reduced seizures by only a small amount in most patients. 

Third is the available evidence on side effects: In the best-done study, 79 percent of participants reported adverse events from cannabis oil, including diarrhea and fatigue, but only 3 percent of them dropped out of the study. That adverse event rate is not small; according to a letter in the journal Lancet Neurology, it's higher than the side effect rate for other epilepsy drugs. But by most expert accounts, the low drop-out rate suggests cannabis oil is safe—for short term use, at least.

For longer-term use, there are some causes for concern—namely that studies suggest extended or chronic cannabis consumption can cause lasting harm to the developing brain; it's at least possible that those harms might be as pronounced for CBD as they are for traditional forms of the drug. 

Still, treatment-resistant epilepsy is a debilitating condition that can dramatically impede a person's quality of life. In severe cases, it can be life-threatening. 

So with those caveats in mind, the bottom line is this: If you live somewhere you can legally obtain CBD, are suffering from intractable seizures, and all other approved therapies have failed, you might want to try cannabis. But you should only do so under the supervision of your regular doctor.