Efforts to get a medical marijuana spray approved as a drug in the U.S. suffered a setback when GW Pharmaceuticals (NASDAQ: GWPH) reported the results of a clinical trial showing that Sativex worked no better than a placebo in relieving cancer pain.

Sativex is composed primarily of two cannabinoids: CBD (cannabidiol) and THC (delta 9 tetrahydrocannabinol), which are administered in an oral spray. Sativex is already being sold in Europe, Canada and Mexico to treat muscle spasticity caused by multiple sclerosis. Canada also allows Sativex to be used for relief of neuropathic pain and advanced cancer pain.

The spray is not currently approved for use in the U.S. for any condition, but is getting a “fast track” review from the FDA as a treatment for cancer pain. It is estimated that 420,000 cancer patients in the U.S. suffer from pain that is not well controlled by opioid pain medications.

“Unfortunately in this initial Phase 3 trial, Sativex did not meet the primary end point of demonstrating a statistically significant difference from placebo, as assessed on the patient assessment of pain using a 0 to 10 numeric rating scale,” said Justin Gover, GW’s CEO in a conference call with analysts.

The randomized double-blind placebo-controlled study recruited 399 cancer patients on opioid pain medication in the U.S., Mexico and Europe. They were given Sativex or a placebo at a dose range of 3-to-10 sprays per day over a 5-week treatment period. Patients remained on opioid therapy during the study. Sativex was well tolerated, with the only side effects in some patients being dizziness and somnolence.

This was the first of three Phase 3 trials of Sativex carried out by GW in collaboration with Otsuka Pharmaceutical. The companies are still hopeful that positive results from the other two trials will enable them to submit a new drug application (NDA) to the FDA.

“Although today’s data is clearly a setback, we believe that if these two additional Phase 3 trials are positive, the data set would allow us to submit an NDA with the FDA and as such we look forward to the data from these studies later in the year,” said Gover.

Results from a second pivotal trial are expected in the second quarter of 2015. GW and Otsuka are in the process of conducting a third Phase 3 trial, which is expected to enroll approximately 540 patients and is designed to provide additional information on the effects of Sativex in treating opioid-resistant cancer pain.

“We believe that cannabinoid therapy offers a potentially novel approach as a co-analgesic to provide pain relief beyond opioid therapy. Too many patients with advanced cancer do not attain adequate pain relief from an opioid regimen, or experience unacceptable opioid side effects,” said Dr. Marie Fallon, a professor of Palliative Care at University of Edinburgh and the principal investigator.

“Whilst I am naturally disappointed that this first trial did not achieve its primary endpoint, I remain optimistic about the potential of Sativex and look forward to the upcoming data from the remaining Sativex Phase 3 trials later this year.”