Preliminary clinical trials show marijuana might be useful for pain, nausea and weight loss in cancer and HIV/AIDS and for muscle spasms in multiple sclerosis. Medical marijuana studies in the U.S. are dwindling fast, however, as funding for research in California—the only state to support research on the whole cannabis plant—comes to an end this year and federal regulations on obtaining marijuana for study remain tight.

In July the Drug Enforcement Administration denied a petition, first filed in 2002 and supported by the American Medical Association, to change marijuana’s current classification. So marijuana remains in the administration’s most tightly controlled category, Schedule I, defined as drugs that “have a high potential for abuse” and “have no currently accepted medical use in treatment in the U.S.” Many medical cannabis proponents see a catch-22 in the U.S.’s marijuana control. One of the DEA’s reasons for keeping marijuana in Schedule I is that the drug does not have enough clinical trials showing its benefits. Yet the classification may limit research by making marijuana difficult for investigators to obtain.

Even as prospects for whole-plant marijuana research dim, those who study isolated compounds from marijuana—which incorporates more than 400 different types of molecules—have an easier time. The drug’s main active chemical, delta 9-tetrahydrocannabinol (THC), is already FDA-approved for nausea and weight loss in cancer and HIV/AIDS patients. The Mayo Clinic is investigating the compound, trade-named Marinol, as a treatment for irritable bowel syndrome. Researchers at Brigham and Women’s Hospital in Boston are studying Marinol for chronic pain.

Compared with smoked or vaporized marijuana, isolated cannabis compounds are more likely to reach federal approval, experts say. Pharmaceutical companies are more likely to develop individual compounds because they are easier to standardize and patent. The results should be similar to inhaled marijuana, says Mahmoud ElSohly, a marijuana chemistry researcher at the University of Mississippi, whose lab grows the nation’s only research-grade marijuana.

Other investigators say a turn away from whole-plant research would shortchange patients because the many compounds in marijuana work together to produce a better effect than any one compound alone. Inhaling plant material may also provide a faster-acting therapy than taking Marinol by mouth. While ElSohly agrees that other marijuana compounds can enhance THC, he thinks just a few chemicals should re-create most of marijuana’s benefits.