PORTLAND — Cannabis products containing THC does relieve chronic pain, at least in the short-term, a new study reveals. The report is one of the first into the pain-relieving properties of marijuana, despite its legalization across several states in the U.S.

Over 20 states have legalized medical and recreational marijuana, however, researchers from Oregon Health & Science University (OHSU) found many products available in dispensaries have not been studied.

In the federally funded review, published in the Annals of Internal Medicine, researchers went through 3,000 prior studies and found a total of 25 with scientifically valid evidence – including 18 randomized controlled studies and seven observational studies of at least four weeks.

Pain relievers still come with side-effects

They also found there was evidence to support a short-term benefit in treating neuropathic pain — such as diabetic neuropathy, which causes a burning or tingling sensation when the nerves suffer damage. The drugs found to be effective included tetrahydrocannabinol, or THC, and dronabinol (Marinol) with nabilone (Cesamet). Both products also led to notable side-effects including sedation and dizziness, according to the review.

Another product, an under-the-tongue spray known as nabiximols, made of equal parts THC and cannabidiol, or CBD, also appeared to treat neuropathic pain. This also led to side-effects such as nausea, sedation, and dizziness.

“In general, the limited amount of evidence surprised all of us,” the study’s lead author Professor Marian McDonagh says in a university release. “With so much buzz around cannabis-related products, and the easy availability of recreational and medical marijuana in many states, consumers and patients might assume there would be more evidence about the benefits and side effects.”

“Unfortunately, there is very little scientifically valid research into most of these products,” McDonagh adds. “We saw only a small group of observational cohort studies on cannabis products that would be easily available in states that allow it, and these were not designed to answer the important questions on treating chronic pain.”

“For some cannabis products, such as whole-plant products, the data are sparse with imprecise estimates of effect and studies had methodological limitations,” the researchers say.

Mimicking the body’s own ability to manage pain

Cannabis products are based on their ability to mimic the body’s own endocannabinoid system. This is comprised of receptors and enzymes in the nervous system that regulate bodily functions and can affect the sensation of pain.

In the evidence review, researchers sorted the types of products into high, comparable, and low ratios of THC to CBD and compared their reported benefits and side-effects. Dronabinol and nabilone fit into the high THC and CBD ratio category, with 100 percent THC (no CBD) showing the most benefit among the products studied. Six of the randomized controlled studies demonstrated statistically valid benefits for easing neuropathic pain compared to a placebo.

“Cannabis products vary quite a bit in terms of their chemical composition, and this could have important effects in terms of benefits and harm to patients,” says co-author Dr. Roger Chou. “That makes it tough for patients and clinicians, since the evidence for one cannabis-based product may not be the same for another.”

The living review includes a visual abstract summary of the findings and will also be shared on a new web-based tool launched by OHSU and VA Portland Health Care System this year to help clinicians and researchers evaluate the latest evidence on the health effects of cannabis.

Known as Systematically Testing the Evidence on Marijuana, or STEM, the project includes clinical briefs to help health care workers translate the clinical implications.

“This new living evidence review is exactly the type of resource clinicians need to clarify for patients the areas of potential promise, the cannabis formulations that have been studied and, importantly, the major gaps in knowledge,” notes co-author Dr. Devan Kansagara.

“Honestly, the best advice is to talk to your primary care physician about possible treatments for chronic pain,” McDonagh concludes. “If you want to consider cannabis, you need to talk to your doctor.”

South West News Service writer Danny Halpin contributed to this report.