To investigate the real-world effects of marijuana, however, researchers need a product that looks and feels like the real thing. And they're increasingly frustrated with government weed that is something else entirely.

Don't take their word for it. The photo below shows a sample of federal marijuana distributed to Sue Sisley, a researcher who just embarked on a first-of-its-kind clinical trial to test the efficacy of medical marijuana for military veterans suffering from PTSD.

Here they are side by side:

A quick glance confirms it looks nothing like the commercial marijuana depicted above. While the real stuff is chunky and dark green, the government weed is stringy and light in color. It appears to be full of stems, which most consumers don't smoke. “It doesn’t resemble cannabis. It doesn’t smell like cannabis,” Sisley told PBS NewsHour last week.

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“In two decades of smoking weed, I've never seen anything that looks like that,” Browne said. “People typically smoke the flower of the plant, but here you can clearly see stems and leaves in there as well, parts that should be discarded. Inhaling that would be like eating an apple, including the seeds inside it and the branch it grew on.”

It's unclear if this is an exceptionally bad batch, but there's reason to strongly suspect it's typical of what most researchers are given.

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All federal marijuana is grown at a single facility at the University of Mississippi, overseen by the National Institute on Drug Abuse (NIDA). Last summer the DEA formally took steps to allow other entities to supply marijuana for research purposes. So far, none have been approved.

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The problems with the Mississippi weed go well beyond aesthetics.

For instance, the pot grown there maxes out, potency-wise, at about 13 percent THC (the main chemical that gets you high). And that might be an overstatement — Sisley's own testing found that one of NIDA's strains purported to be 13 percent THC was actually closer to 8 percent.

By comparison, the typical commercial weed available in Colorado is at about 19 percent THC, according to a laboratory that tests commercial marijuana in the state. And that's just the average — some of the higher-end strains are pushing 30 percent THC or more.

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For a researcher, it's difficult to assess the real-world impact of high-end pot if you only have access to the low-quality stuff. It's akin to investigating the effects of bourbon by giving people Bud Light.

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For certain types of research this isn't necessarily a problem, says Rick Doblin, founding director of the Multidisciplinary Association for Psychedelic Studies, a group that's been working with Sisley on the PTSD trial. "[NIDA's] marijuana is fine if you want to do academic research,” Doblin said — studies that look at how marijuana affects the body in a laboratory setting, for instance.

But NIDA's weed doesn't pass muster if you want to know how marijuana use is affecting people in the real world. Or if you want to run highly controlled medical experiments, like the one Sisley and Doblin are working on. It's not even tested for some common contaminants, like yeast and mold, that many states now check for as part of their regulatory regimes.

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Doblin said the marijuana they received from NIDA showed levels of mold and yeast that far exceeded standards for some states, like Colorado and Washington. Be they opted to go ahead with the trial since additional testing confirmed that none of the strains of mold and yeast found in the plant material posed a risk to humans.