Cannabis products containing the active ingredient THC do relieve chronic pain in the short term, scientists have revealed.

The study is one of the first into its pain-relieving properties despite many products being legalized and sold across the U.S.

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

In the federally funded review, researchers trawled through 3,000 studies in the scientific literature up to January this year and found a total of 25 with scientifically valid evidence—18 randomized controlled studies and seven observational studies of at least four weeks.

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 become damaged.

The drugs found to be effective were tetrahydrocannabinol, or THC, and dronabinol (brand name 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.

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% THC (no CBD) showing the most benefit among the products studied.

And 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 side effects in terms of benefits and harm to patients," co-author Dr Roger Chou, at OHSU, said. "That makes it tough for patients and clinicians alike since the evidence for one cannabis-based products 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," co-author Dr Devan Kansagara, also of OHSU, said.

"Honestly, the best advice is to talk to your primary care physician about possible treatments for chronic pain," Dr McDonagh added.

This review is published in the Annals of Internal Medicine.

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