A group of scientists just published an article claiming the US government is holding back proper research into marijuana, due to its overzealous regulation of the substance.

While millions of patients are already using medical marijuana - and more and more states are decriminalising the drug - scientists are still denied access to high-quality weed, which makes it hard to properly investigate the risks and benefits of consuming the plant.

"This has created a truly unique and an unnecessary paradox in modern medicine, in which physicians are authorising treatments to patients, and patients are regularly using medication without a scientific basis of knowledge on patient outcomes, forced rather to rely only on scientifically invalid or anecdotal information," say Sarah Stith and Jacob Vigil from the University of New Mexico.

In a letter published in Science, the researchers claim that while we have compelling evidence that cannabis has a whole range of therapeutic benefits - users report it's capable of easing symptoms of chemotherapy, chronic pain, and post traumatic stress disorder - many of these effects haven't been tested in externally valid, randomised clinical trials.

That's because the US government continues to list Cannabis sativa as a Schedule 1 drug - a category limited to drugs with "no currently accepted medical use" and "high potential for abuse" - despite the fact that more than 75 percent of US physicians polled think medical cannabis is a safe and effective treatment.

Because of its classification, researchers can only access cannabis through the National Institute on Drug Abuse (NIDA) - and NIDA only supplies product that's significantly less potent than the weed available on the street.

"Millions of patients have been granted the authorisation to use medical cannabis and cannabis-based products by their respective state health departments and four states have begun taxing and regulating cannabis sold for 'recreational' purposes," said Vigil and Stith.

"However, the federal government continues to categorise Cannabis sativa as a Schedule I drug under the Controlled Substances Act, a more restrictive categorisation than that used for cocaine, methamphetamine and PCP."

The big problem with this NIDA-controlled drugs is that, despite multiple proposals from other universities, the government only supplies cannabis grown at the University of Mississippi. And their plant is a lot weaker than the product prescribed by doctors.

For example, the highest level of tetrahydrocannabinol (THC) - the main psychoactive ingredient in cannabis - available to researchers through NIDA is 12.4 percent, the researchers report. Last year, the only two clinical studies funded by the National Institute of Health (NIH) used products with potency levels between 3.5 and 7 percent THC.

To put that into perspective, the medical cannabis now sold in Colorado averages 18.7 percent, with some strains as strong as 35 percent THC. Some states, including New Mexico, have given permission for edibles and vaping products to contain up to 70 percent potency.

"Clearly, results from studies using Cannabis sativa obtained from the University of Mississippi offer little to no insight into the effects actually experienced by medical marijuana patients in terms of both therapeutic benefits and negative side effects, if any," say Vigil and Stith.

The researchers call for government control over cannabis research to start easing up alongside their regulation of medical marijuana, so scientists can make sure everything available to the public is actually safe and effective.

"As long as clinical research on cannabis is controlled by regulators expressly opposed to any increase in its consumption, health care cost reductions may be missed, and intoxication and long-term effects will remain unknown," they write.

"Most important, many severely ill patients may suffer unnecessarily because no one knows the true risks and benefits of consuming Cannabis sativa."