A new class of epilepsy medications based on an ingredient derived from marijuana could be available as soon as the second half of 2018 in the United States, pending Food and Drug Administration approval.

Officials from GW Pharmaceuticals, the company that developed the drug, on Wednesday announced promising results from a study on 171 patients randomized into treatment and placebo groups. Members of the group, ages 2 to 55, have a condition called Lennox-Gastaut syndrome and were suffering from seizures that were not being controlled by existing drugs. On average they had tried and discontinued six anti-seizure treatments and were experiencing 74 “drop” seizures per month. Drop seizures involve the entire body, trunk or head and often result in a fall or other type of injury.

The results, published in the Lancet, show that over a 14-week treatment period, 44 percent of patients taking the drug, called Epidiolex, saw a significant reduction in seizures, compared with 22 percent of the placebo group. Moreover, more of the patients who got the drug experienced a 50 percent or greater reduction in drop seizures.

Elizabeth Thiele, director of pediatric epilepsy at Massachusetts General Hospital and lead author of the study, said the results varied depending on the patient.

“For some, it does not do a whole lot. But for the people it does work in, it is priceless,” she said.

“One child who comes to mind had multiple seizures a day. She had been on every medication possible,” said Thiele, a professor of neurology at Harvard Medical School. Then the patient tried the cannabis-based treatment and has been seizure-free for almost four years. “She is now talking about college options. She would have never had that conversation before. It has been life-changing.”

A town councilor in Standish, Maine, made headlines when he posted a video of his son having a seizure who appears to calm down after a substance is rubbed on his gums. Peter Starostecki said he and his family moved to Maine because medical marijuana helps control his son’s seizures and is legal. The Facebook post captured more than 13 million views.

Epidiolex is based on a purified cannabidiol, an oil from marijuana plants that has made headlines in recent years.

Interest in the substance, one of many components of marijuana plants, began to take off when Paige Figi, a Colorado mom, began writing about how an extract – nicknamed “Charlotte’s Web” from the special strain it was taken from – had nearly stopped her daughter’s seizures. Numerous families moved to the state, where marijuana is legal, to try to get the treatment, and others mobilized in their own states to try to get permission to import it.

Despite its apparent effect, Charlotte’s Web has been controversial because of the lack of oversight and regulation regarding the production, sales and dosaging. The oil was produced by a small business run by brothers who are cannabis growers. Epidiolex, which is based on the same scientific theory about cannabidiol as an active ingredient but is unrelated to Charlotte’s Web, is intended to be a prescription drug dispensed by doctors.

“As a pharmaceutical product, it is the subject of rigorous and intense manufacturing controls. Patients have the reassurance that the product is what we say it is and what they think it is,” Justin Gover, GW’s chief executive officer, said in an interview. He declined to discuss the pricing of the drug but said the company is already in discussions with health insurers about coverage.

A previous study, published in the New England Journal of Medicine in May, showed that Epidiolex also appears to be safe and effective in children who suffer from Dravet syndrome, a rare, severe form of epilepsy.

Epidiolex is somewhat unusual in the world of drugs under development because about 1,500 people are already taking the drug free under the FDA’s “compassionate use” exception, which allows products to be used on a limited basis before they are approved.

GW Pharmaceuticals said it also has a new drug application pending before the European Medicines Agency.