Until he was two years old, Liam O’Brien was a typical toddler—happy, active, and curious about the world. He would trot around his family’s home on Staten Island, leaving a trail of toys and exercising his growing vocabulary.

Then, one day in June 2011, Liam did something strange: He walked into the kitchen, dropped his chin to his chest, and thrust one of his arms in the air. He stood motionless for several seconds before continuing on his way.

In the weeks that followed, such episodes became more frequent. Liam also began losing his language skills. His worried parents, Karyn and Kevin, took him to a pediatric neurologist, who diagnosed him with autism. An electroencephalogram showed that he also had epilepsy, a condition in which abnormal bursts of electrical activity interfere with normal brain function. The resulting seizures can take many forms, from a brief dimming of consciousness to body-racking convulsions. Liam was experiencing a type known as myoclonic seizures, which sometimes cause involuntary movements in just one area of the body.

The neurologist started him on an anticonvulsant medication, but it had little effect. A second drug was added, but the seizures multiplied. They came in clusters, totaling 50 or 60 a day.

Seeking a team of specialists who could handle Liam’s complex case, his parents brought him to NYU Langone’s Comprehensive Epilepsy Center to meet with its director, Orrin Devinsky, MD, professor of neurology, neurosurgery, and psychiatry. One of the country’s leading epilepsy specialists, Dr. Devinsky has led many major studies of novel treatments. He ordered a thorough workup and prescribed a regimen of targeted medications. Although they reduced the frequency of the seizures, new problems arose. Liam began having atonic seizures, suddenly losing all muscle tone and crashing to the floor. He also developed tonic seizures, in which his entire body stiffened; often, he stopped breathing for 30 seconds or longer. “It was so scary,” Karyn recalls. “I never knew if he was going to start breathing again.”

As the months wore on, Liam was given more than half a dozen medications, but none eased his symptoms. Moreover, the drugs brought troubles of their own, making him floppy limbed and sleepy. The combined effects of the drugs and the seizures, which disrupt learning and may cause brain damage, added to the developmental delays associated with his autism. By age four, Liam could speak fewer than 10 words and was unable to use a toilet. To avoid injury, he was forbidden to climb stairs unassisted or sit in a chair without arms. He had to wear a helmet at all times to prevent concussions when he fell.

Liam’s condition took a toll on his parents as well. His nighttime seizures left them perpetually sleep deprived. Any family outing could turn into an ordeal. Letting him play unsupervised was unthinkable. Kevin worked as a computer programmer, but Karyn’s full-time job was making sure their son survived each day.

In July 2013, a TV documentary gave her a sliver of hope. Its subject was a little girl in Colorado who suffered from severe epilepsy and had received little benefit from conventional treatments. But the girl’s seizures seemed to have been all but banished by a highly unconventional remedy: an extract of cannabis. Along with a handful of other children with similar conditions, she used a strain that was high in cannabidiol (CBD), the plant’s main nonpsychoactive chemical component, and low in intoxicating tetrahydrocannabinol (THC).

Karyn knew there were reasons to be wary. Possession of marijuana was a felony under federal law, and though Colorado had legalized medical marijuana, New York had not yet done so. But she was willing to try anything. “We can’t live like this anymore,” she told Dr. Devinsky’s nurse practitioner, Erin Conway, RN. “I’m ready to move to Colorado.”

Conway suggested a less radical move. Dr. Devinsky, she said, was about to launch the first large-scale study of CBD as a treatment for severe, treatment-resistant epilepsy, and Liam might be eligible.

Dr. Devinsky had begun looking into CBD after another patient’s father asked him about reports in the national media of supposed miracle cures wrought by extracts containing the substance. While there was very little in the literature about marijuana’s anticonvulsant effects in humans, the neurologist was encouraged by the research in animals. Data showed that two of the most plentiful of the plant’s 80-plus unique compounds, known as cannabinoids, could quell seizures—THC in 61 percent of animal studies, and CBD in 81 percent. THC interacts primarily with the endocannabinoid system, a neurological network that helps regulate physiological processes including sleep, memory, mood, appetite, inflammation, immune function, and bone growth. (The body makes chemicals, known as endocannabinoids, which transmit messages through the system by binding to two types of receptors—CB1, found largely in the brain, and CB2, found mainly in other organs. THC fits into those receptors like a key into a lock.) CBD, meanwhile, works mainly with other signaling systems. Its anticonvulsant effects seem to involve receptors that help regulate the excitability of brain cells by altering the balance of calcium ions, and unlike THC, it doesn’t deliver a high.

By studying the potential of cannabinoids for treating epilepsy, researchers hope to separate truth from rumor.

Dr. Devinsky was intrigued enough by CBD to begin exploring his own study. But there was one big catch: the compound was available only in the form of homemade oils and tinctures, with little control over consistency or purity. Then Dr. Devinsky heard about GW Pharmaceuticals, a British company that manufactured a cannabis-based medication, Sativex, which was approved in 22 European countries for relief of multiple-sclerosis spasticity. The firm’s manufacturing processes were cutting-edge, and its product—an extract standardized to 50 percent THC and 50 percent CBD—met strict pharmaceutical standards. Dr. Devinsky called the company’s CEO in London to explain what he had in mind.

Soon afterward, GW representatives sat down with researchers from NYU Langone and several other institutions in a conference room at the Medical Center’s Comprehensive Epilepsy Center. Together, they hatched a plan for a multicenter trial of a purified CBD extract.

Epilepsy is not a single disorder, but rather a group of disorders characterized by recurrent seizures that have no immediate cause. The condition, which afflicts about 1 percent of the population, occurs when the brain’s electrochemical balance is disrupted, allowing neurons to fire excessively and for too long. In a seizure, a group of overexcited brain cells begin firing in an abnormally synchronized way. This sets off a storm that can involve just part of the brain (a partial seizure) or both hemispheres (a generalized seizure). Some rare types of epilepsy result from a defect in one gene. Other varieties can be triggered by brain injury, stroke, tumors, or substance abuse. Population studies indicate that heredity is a factor, but in most cases, the underlying cause is unknown. In about half of those affected, the disease can be controlled by a single medication. (More than two dozen types of anticonvulsants are available.) Around 30 percent of patients continue to have seizures, however, even when taking two or more drugs at once. Up to 25 percent experience adverse effects that limit treatment options. For some patients with severe epilepsy who don’t respond to any treatments, removal of malfunctioning brain areas, though a last resort, is a viable option. Another is implanting a pacemaker to stimulate the thalamus (a structure deep within the brain) or vagus nerve. Such surgical procedures, however, can also lead to serious complications.

Caring for a child with a severe, chronic disease is always difficult, but because intractable pediatric epilepsy is usually associated with cognitive and behavioral problems, the challenges can be particularly daunting. Kids with epilepsy also have a four fold higher risk of death, according to the Centers for Disease Control and Prevention. Yet parents desperate for alternatives to conventional treatments will sometimes expose their children to other kinds of dangers. “They may try remedies they’ve seen on the news or on the Internet but that haven’t been objectively proven to be helpful,” observes Judith Bluvstein, MD, assistant professor of neurology and codirector of Pediatric Special Procedures at NYU Langone. “In some cases, such treatments may actually do more harm than good.”

By studying the potential of cannabinoids for treating epilepsy, researchers hope to separate truth from rumor and to develop new treatments that are safer and more reliable than any do-it-yourself nostrum. Yet the hurdles facing scientists who wish to study cannabis and its active ingredients remain discouragingly high. The U.S. Controlled Substances Act, first enacted in 1970, stipulates five “schedules,” or levels of restriction, based on a drug’s degree of danger and medical value. Cannabis is classified as Schedule I—a category for drugs such as heroin and LSD, with a high potential for abuse, no accepted medical use, and no standard for safe use. Possession of the plant or its byproducts, except by specially licensed individuals, is punishable by prison and heavy fines.

Since 1996, when California became the first state to legalize medical marijuana, 22 others have followed. (New York’s statute, passed in July 2014, is more restrictive than many but includes epilepsy among conditions legally treatable with cannabis.) Meanwhile, the drug’s outlaw status on the federal level has increasingly been called into question. “Schedule I is crazy,” Dr. Devinsky says bluntly. In recent years, the American Medical Association and more than 30 other mainstream medical groups have asked the government to consider reclassification. They cite cannabis’s relatively low addictive power (9 percent of users get hooked, versus 32 percent of cigarette smokers) and demonstrated ability to ease symptoms stemming from chemotherapy, HIV, and other serious conditions, as well as growing evidence that cannabinoids could help control—or even cure—many ailments. As the AMA’s position paper puts it, the fact that marijuana is also used for getting high “does not obviate its potential for medical product development.”

Nonetheless, researchers continue to navigate a grueling legal obstacle course. In addition to FDA approval, researchers must get permission from the Drug Enforcement Administration and state drug-control agencies. Marijuana must be obtained through the National Institute on Drug Abuse, which grows a small crop at the University of Mississippi. As mandated by law, the plant and its derivatives—even nonpsychoactive CBD—must be stored in an extra-heavy safe with an elaborate alarm system. “Federal oversight essentially handcuffs cannabis researchers in the U.S.,” explains Dr. Devinsky. “Other countries are far ahead of us.”

Despite all this, Dr. Devinsky and colleagues managed to launch the 10-center study in January 2014, using a new GW Pharmaceuticals product called Epidiolex, an orally administered extract containing 99 percent CBD. The initial report, released in March of this year, provided data for 123 patients who had been treated for at least 12 weeks. The results were promising: Total seizures showed a median reduction of 46 percent. For patients with Dravet syndrome—a devastating and often treatment-resistant form of epilepsy—the reduction was 52 percent. Overall, 10 percent of patients—and 22 percent of Dravet’s patients—were seizure-free. Adverse effects compared favorably with many conventional epilepsy medications: 21 percent of patients experienced sleepiness, 17 percent diarrhea, 17 percent fatigue, and 16 percent loss of appetite.

Does that confirm CBD’s growing reputation as a miracle drug? Hardly. The study was mostly intended to establish parameters for further research. It was “open label,” meaning that everyone involved knew what patients were getting, and there was no control group receiving a placebo. Nor does CBD appear to be strikingly superior to other anticonvulsants, aside from its relatively benign side-effect profile. “There are some patients who benefited, some who didn’t, and some who actually worsened,” notes epileptologist Daniel Friedman, MD, who helped lead the study. Because so little is known about CBD’s effects, all patients in the study continued taking their existing medications, leading, in some cases, to problematic drug interactions.

Still, the prospect of having another weapon in the antiepileptic arsenal is encouraging to many. Dr. Devinsky’s team recently began a double-blind, placebo-controlled study of Epidiolex for children with Dravet’s and another genetic form of severe epilepsy, Lennox-Gastaut syndrome. He hopes that CBD will eventually improve the lives of large numbers of epilepsy sufferers—adults as well as children. Meanwhile, he’s pleased that that the drug has helped at least a few of his patients. “Nothing makes me happier than seeing people get better,” he says. “It’s very gratifying.”

Liam O’Brien was among the lucky 10 percent, though it took a week for the results to become clear. He got his first dose of Epidiolex on February 7, 2014, and remained at NYU Langone’s Comprehensive Epilepsy Center for three hours afterward to be monitored for adverse effects. None appeared, but he had a seizure later that day. On February 8, he had three seizures, the following day, none. There were several seizures on February 10 and 11, none on February 12, and one each day on February 13 and 14. He hasn’t had a seizure since.

Liam is six years old now, a round-cheeked boy with a blond crew cut and lively blue eyes. Those terrifying episodes of falling to the floor, unable to breathe, are gone. His autism hasn’t vanished, but he’s far more engaged with his surroundings. He finally mastered the potty. He can say more than 100 words, including his name, and form short sentences (“I want iPad” is a favorite). He no longer needs a helmet for most activities. He even learned to ride a bike. “I can relax a little bit,” says Karyn O’Brien. “I can actually sit down and just let him be a kid.”