GW Pharmaceuticals (GWPH) is already well on its way to winning the first-ever U.S. approval for a cannabis-derived therapy. But an early trial suggests that these treatments could also be an effective way to fight one of most devastating forms of brain cancers: glioblastoma multiforme.

The U.K.-based company unveiled preliminary data Tuesday from a mid-stage study on an experimental drug combining cannabidiol and THC, the "high" producing element of marijuana. Results so far show that the drug boosted brain cancer patients' median survival rates by about six months compared to a placebo. Typically, this type of cancer ravages the brain and (on average) leaves 70% of patients dead within two years of being diagnosed.

"We believe that the signals of efficacy demonstrated in this study further reinforce the potential role of cannabinoids in the field of oncology and provide GW with the prospect of a new and distinct cannabinoid product candidate in the treatment of glioma," GW CEO Justin Gover said in a statement.

GW is already interpreting the results as a reason to expand its foray into cancer treatment. The company's most advanced drug candidate, Epidiolex (for treatment of severe epilepsy related to a number of rare disorders), is closest to reaching the U.S. market. But the firm has staked out more far-reaching ambitions in an environment where cannabis-based products have been increasingly accepted.

For one, Gover thinks that cannabidiol-based therapies show plenty of promise in behavioral disorders like schizophrenia, he told Fortune last year.

Marijuana's effect on cancer still isn't all that clear. A big recent review by American scientists suggests the drug is effective for treating pain and nausea in cancer patients but doesn't necessarily treat (or cause) cancer. However, GW's drug isn't just a bowl of weed to be smoked - it contains concentrated derivatives and is undergoing the kind of clinical testing that could provide insights hampered by U.S. policy towards studying cannabis.