WASHINGTON (AP) -- An experimental medication made from marijuana successfully reduced back pain in a new study, offering further support for the drug's potential in treating one of the most common forms of chronic pain.

The 800-patient study by a German drugmaker is the latest evidence of the therapeutic properties of cannabis, which remains illegal under U.S. federal law even as most states have made it available for medical or recreational use.

Health officials in Canada and Europe have previously approved a pharmaceutical-grade form of cannabis for several types of pain, including nerve pain due to multiple sclerosis. In the U.S., the Food and Drug Administration has approved a drug containing CBD -- one of the many non-intoxicating chemicals found in cannabis -- to treat rare seizures in children with epilepsy.

Unlike that drug, known as Epidiolex, the new cannabis formula from drugmaker Vertanical contains THC, the active ingredient in marijuana that gets users high. But levels of the chemical are very low, essentially a microdose compared to what's available in gummies, chocolate bars and other products sold at marijuana dispensaries in the U.S. The company said patients in the trial didn't show any signs of drug abuse, dependence or withdrawal.

Vertanical is seeking approval for a large group of patients: those suffering from lower-back pain, a chronic condition that affects millions and has few proven treatments.

Over-the-counter pain relievers like ibuprofen can't be used for long-term pain because of their side effects, which include stomach ulcers and indigestion. Opioids are no longer recommended, after the overprescribing of painkillers such as OxyContin in the 1990s and 2000s led to the ongoing epidemic of addiction to that class of drug.

Chronic pain is one of the most frequently cited conditions of people enrolled in state-run medical marijuana programs. But there's been little rigorous research on the drug's use in that group.

Lead study author Dr. Matthias Karst said in an email that the new findings show cannabis "can significantly reduce pain and improve physical function in patients with chronic low-back pain, without the safety concerns commonly associated with opioids." Karst is a pain specialist at Hannover Medical School and a consultant for Vertanical.

For the new study, patients with back pain were randomly assigned to take Vertanical's proprietary liquid cannabis extract or a placebo.

At the end of 12 weeks, patients taking the medication reported a nearly 2-point reduction in pain on an 11-point scale, compared with 1.4 points for those taking placebo. The difference was statistically significant. Those getting the drug also reported improvements in sleep and physical function.

Patients who continued with a six-month extension phase continued to experience reductions in pain. The results were published Monday in the journal Nature.

Side effects included dizziness, headache, fatigue and nausea and led to more than 17% of people discontinuing the drug early. Researchers said that dropout rate was lower than what's typically reported with opioids, which can cause constipation, nausea, drowsiness and carry risks of addiction.

Vertanical has filed an application for its drug with European regulators. In the U.S., the company says it is "working closely" with regulators to design a study to support FDA approval.

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