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A small, new study backs a long-standing claim of advocates of medical marijuana: pain patients can safely use cannabis while taking opioid painkillers, and may actually need fewer pills because of it.

The research included 21 chronic pain patients, who were taking either long-acting morphine or Oxycontin twice a day. Adding marijuana to these opioid drugs reduced patients’ pain by an average of 27% and did not significantly affect blood levels of the prescription drugs. If marijuana had raised those blood levels, it could have increased overdose risk.

“The combination may allow for opioid treatment at lower doses with fewer side effects,” the authors concluded.

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For the duration of the study, participants were housed and monitored in the hospital for five days, so that their vital signs could be tracked and any complications rapidly treated. They were given vaporized cannabis to inhale: the first day, they used it in the evening; for the next several days, they smoked three times a day; and on the last day, they used it in the morning.

No medical problems occurred. The patients did report feeling “high” when given the marijuana, but not when given opioid drugs, to which they were already tolerant. The study was not placebo-controlled so it’s possible that some of marijuana’s painkilling effect was due to patients’ expectations of the drug— but previous research has found that marijuana is superior to placebo as a pain reliever.

With rising concerns about opioid overdose — the death toll associated with prescription painkillers has recently surpassed that of traffic accidents — the new finding is striking. While marijuana will never be able to replace opioids for the most severe pain, it carries no overdose risk and a far lower risk of addiction than prescription painkillers do.

Further study to determine which patients would benefit most, and to what extent marijuana can actually reduce opioid use should be conducted. Unfortunately, the Obama administration is currently engaged in a crackdown on marijuana dispensaries in the 16 states that have legalized medical use. Researchers report difficulty getting funding to do these types of studies.

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Moreover, many prescribers of opioid medications view any marijuana use as substance abuse and refuse to care for people who use both, presenting a barrier to pain patients who would like to try marijuana to reduce their reliance on opioids.

Last year, the Veterans Administration dropped its policy of prohibiting opioid prescribing to medical marijuana patients in states where it is legal. But even if private physicians wish to permit medical marijuana use among pain patients, it poses a legal risk. Cannabis use can be construed as a “red flag” that was missed, if the patient turns out to be misusing drugs. Doctors can be criminally prosecuted if they are fooled by such patients.

Considering the risks posed by the two types of drugs, the current legal situation makes no sense. Cracking down on medical marijuana while trying to fight overdose is not only a waste of money, but it may also be, as the new study suggests, actively counterproductive.

The research was published in Clinical Pharmacology and Therapeutics.

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Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.