Image source: GW Pharmaceuticals plc.

What: Despite reporting encouraging data regarding a marijuana-derived medicine for epilepsy, shares in GW Pharmaceuticals plc (NASDAQ:GWPH) slid 17.5% last month, according to Capital IQ.

So what: Marijuana drug research was dealt a series of blows at GW Pharmaceuticals in 2015 that included the high-profile failure of the THC based drug Sativex for cancer pain and disappointing results for a CBD-based drug for schizophrenia.

The inability to deliver positive efficacy in placebo-controlled studies in these indications trumped somewhat encouraging epilepsy data reported by the company last month.

Specifically, on December 7, GW Pharmaceuticals highlighted physician reports of fewer seizures in epilepsy patients receiving the company's Epidiolex in an expanded access study. Patients in this study were seen to have a median 45% reduction in seizures after 12 weeks of treatment.

Now what: U.K.-based GW Pharmaceuticals is in a horse race with U.S.-based Insys Therapeutics (NASDAQ:INSY), which is also working on a CBD treatment for epilepsy.

Results from GW Pharmaceuticals' and Insys Therapeutics' ongoing phase 3 studies in Dravet syndrome and Lennox-Gastaut syndrome -- two rare forms of the condition -- are expected this year, and if those results are solid, then the two companies could face off against each another in a battle for market share as early as 2017.

However, because the patient population targeted by these marijuana medicines is small, the commercial opportunity -- at least at the onset -- may not be enough to justify GW Pharmaceuticals' current $1.4 billion market cap. For that reason, investors may be better off sitting this one out until data from both companies is reported and there's more insight into who may have an edge, commercially.