NEW ORLEANS -- Use of marijuana to ease back pain was common among patients at a university spine clinic in Colorado where pot has been legal for medical purposes since 2000, but most of the users did not have a prescription, according to research presented here.

Among 184 patients at a Colorado spine center, 19% said they used marijuana for pain relief, but less than half, 46%, actually had a prescription for the drug, according to study co-author Michael Finn, MD, an assistant professor of neurosurgery at the University of Colorado in Denver.

The finding raises concerns about how common marijuana use is around the country for one of the most common medical afflictions and whether it is being used along with narcotic painkillers.

"A lot of states are loosening restrictions, especially for medical use," Finn said during his presentation at the North American Spine Society's annual meeting.

Since being legalized for medical purposes in Colorado in 2000, more than 115,000 patients have been registered to get marijuana from more than 800 dispensaries. Last year, the state legalized marijuana for recreational purposes.

One of the concerns about marijuana is that not much is known about its safety and effectiveness for easing pain caused by spine problems. Most of the patients in the study were suffering from degenerative disc problems.

Finn and colleagues offered some interesting insights into the issue, but also left a lot of questions unanswered.

The research looked at 200 patients at the University of Colorado's Spine Center, including 184 who agreed to participate in the study.

The most common way to use the drug was smoking it, 90%, followed by oral ingestion, 45%, and vaporization, 29%.

The typical user in the study used marijuana no more than one or two times a day, and 83% also were taking other medications, mostly narcotic painkillers.

Side effects were reported by 14% of the users, including depressed mood, difficulty concentrating, memory problems, weight gain, and paranoia.

According to the users, marijuana worked. A total of 89% said it greatly or moderately relived their pain, and 81% said it worked as well as or better than narcotic painkillers.

But Finn said his study just could not answer the question of whether marijuana was effective for back pain relief.

He said the best evidence shows some benefit in pain relief for people with MS and rheumatoid arthritis, but little research exists in the area of back pain.

He said his "gut feeling" is that there may be some benefit for neuropathic back pain such as pain that shoots down the legs or arms, but, he conceded, "I don't have data to back that up."

He said the next step is to do a study based on the types of spine pain and the amounts of marijuana used. He said he also would like to know what happens when marijuana is used along with opioid painkillers.

"Given that one in five patients are using it, there is a real need to look at more," he said.

Christopher Standaert, MD, a clinical professor of rehabilitation medicine at the University of Washington, said he was concerned at the high percentage of marijuana users who also were using opioids.

"It tells you marijuana isn't supplanting the need for other drugs," said Standaert, who was not involved in the study.

At same time, it raises concerns about whether marijuana may be aggravating the effects of opioids.

"We don't have any information on dose response," he said. "It goes back to the biology of it. We don't know exactly what it is treating."

Standaert noted that pain and depression often go hand in hand, including among those who take opioids.

"Opioids are overprescribed," he said. "And now you are throwing another actor into the mix."