The only marijuana researchers can legally obtain for studies looks like something you would scrape off the bottom of your shoe after walking on a grassy field.

This is not an exaggeration. Take a look at this photo, courtesy of the Multidisciplinary Association for Psychedelic Studies (MAPS):

This is the marijuana that researchers were sent for a study looking at whether pot can help treat post-traumatic stress disorder.

Due to federal prohibition and regulations, all of the marijuana used for US research is provided by one facility at the University of Mississippi through the National Institute on Drug Abuse (NIDA). But researchers have complained for years that the quality of marijuana that NIDA supplies is terrible — typically far below what you can get from state-legal medical or recreational marijuana markets or even the black market.

The photo above exemplifies this. The marijuana looks like it’s made up more of leaves and stems than the actual bud you’re supposed to smoke. As anyone who’s ever smoked pot can tell you, you’re typically supposed to throw out the leaves and stems — meaning what you see in the photo is basically garbage to the typical user. Usable pot is supposed to look chunkier and laced with crystals that are high in THC (which is what gets you high).

Here’s an example of higher-quality pot, taken before the stems are fully removed:

But as Christopher Ingraham and Tauhid Chappell reported at the Washington Post, the problem is not solely aesthetic. The NIDA-provided marijuana was supposed to have 13 percent THC content, but the MAPS researchers’ own testing found it was closer to 8 percent. (In comparison, state-legal commercial marijuana is typically at 19 percent but can go up to 30 percent or more.) There were also high mold and yeast levels — far beyond what you’d see in state-legal medical marijuana — but ultimately not the kind of mold or yeast that’s harmful to humans.

Yet thanks to the federally enforced monopoly on pot for research, this is the pot that researchers have to work with.

That makes the research questionable. Will it understate pot’s medical benefits, since this pot provided by the government is far weaker than what patients would actually use in the real world? Given that the government can’t get THC levels right, how can researchers be sure that federally provided marijuana won’t fail in other respects, such as the levels of other crucial chemicals like CBD? And since researchers know that the quality of pot they’ll get from the government is bad, how many give up without even trying?

These kinds of hurdles are why, even as marijuana has been with humans in some form of another for thousands of years, we still have little idea of what marijuana’s benefits and harms truly are.

Our lack of knowledge then helps perpetuate prohibition. One reason that pot remains a highly restricted, fully prohibited substance at the federal level is because there are no large-scale clinical trials proving its medical value and safety. But a major reason for the lack of large-scale clinical trials is that federal prohibition limits researchers’ access to good marijuana — by, for example, letting NIDA claim a monopoly on what kind of pot researchers can use. (If pot was legal, researchers could just obtain the drug from a retailer — much like, say, a researcher looking into alcohol or tobacco could.) This is, in other words, a Catch-22.

It’s possible, the Post reported, that this is a particularly bad strain of NIDA’s marijuana. When I asked about this, the agency told me, “The marijuana provided by the University of Mississippi farm supported by NIDA is dried and frozen before being shipped and hence it will look different from the more commonly familiar dried plant sold in dispensaries. However, this does not impact the chemical constituents found within the plant, including THC levels.”

NIDA did tell the Post that “there has been some emerging interest from the research community for a wider variety of marijuana and marijuana products. … NIDA does plan on growing some additional marijuana this year and harvest some high THC material that will likely be above 13 percent THC.”

But as I reported before, NIDA has been hinting at these kinds of steps for years. And as researchers wait, they have to conduct their studies with some pretty crappy weed.

Watch: How medical marijuana could help combat the opioid epidemic