Sha’Carri Richardson, 21, who would have competed in the women’s 100 meters, was suspended from competing for 30 days after she failed a drug test for the banned substance.

Cannabis is banned by the World Anti-Doping Agency (WADA), which the International Olympic Committee refers to when carrying out its testing process.

WADA was founded in 1999 as an organization aimed at coordinating efforts against doping in sport internationally. It oversees compliance of its own code and cooperates with law enforcement.

The Olympian’s suspension has caused a backlash, including from the Cannabis Council advocacy group, which called the decision “abhorrent.”

Which substances does WADA prohibit?

In order for WADA to include a substance on its list of prohibited substances, it has to meet any two of the following three criteria. It has to have the potential to enhance—or does enhance—sport performance; it represents a risk or a potential risk to the athlete; or it violates the spirit of the sport.

There are some other prohibitions as well, such as substances that mask other prohibited substances or those that aren’t approved for human use.

As such, a substance does not have to be performance-enhancing to be banned from use at the Olympics, and a performance-enhancing effect does not necessarily have to be evidenced.

WADA says this list is reviewed every year in consultation with scientists and comes into force on January 1 of each year.

The inclusion of cannabis on WADA’s prohibited substances list has proved controversial over the years.

WADA lists cannabinoids, which include both natural and synthetic cannabis products, as “prohibited in-competition.” The term “in-competition” means athletes are only tested the day before a competition in which they are scheduled to participate.

Daryl Adair, associate professor of Sport Management at the University of Technology in Sydney, told Newsweek: “In other words, if an athlete is a regular user of illicit drugs between competition, but such a substance does not appear in the body on the day of competition, everything is fine, according to WADA.”

Why is marijuana prohibited?

WADA is not explicit about why the substance is banned, though one study, published by a federal drug abuse researcher and two people linked to WADA in the journal Sports Med. in 2011, set out to support its inclusion.

Citing other studies, it said “much more scientific information” was needed on the matter and that at least one study found cyclist performance was hindered by cannabis use.

However, it also said cannabis had the potential to be performance-enhancing because it could help reduce anxiety and because of a suggestion it could increase oxygenation.

But the conversation goes back decades. Roger Pielke Jr., director of the Sports Government Center at the University of Colorado, published a blog post last week in which he detailed how WADA’s cannabis policy is linked to a push by the U.S. government and its so-called “War on Drugs.”

The “war on drugs” was a term coined by the Nixon administration, which launched a far-reaching campaign against drug use and increased the size and power of drug control agencies. It has been heavily criticized since.

What has the U.S. government said?

Pielke wrote: “The use of international sports organizations to pursue domestic U.S. drug policies was bipartisan. For instance, in 2008 the George W. Bush administration reported to Congress that it had successfully resisted pressures from the international community to remove marijuana from the WADA prohibited list.”

Around the start of the millennium, the U.S. Office of National Drug Control Policy (ONDCP) published a list of what it called “Agency Accomplishments and Significant Actions” between the years of 1993 and 2000.

In one section, it refers to the case of a Canadian snowboarder who won an Olympic gold medal at the Nagano Winter Olympics in 1998 despite having tested positive for marijuana.

The ONDCP said it had become worried about the victory because it could “directly undercut our messages to young people that drug use undermines a child’s opportunity to success.”

The ONDCP said it then began a “wave of efforts” to get the International Olympic Committee, which now works with WADA, to ban marijuana from the games, which was ultimately successful.

The drug control office said that, amid these efforts, “athletes and sports officials at all levels” reached out to state they were more concerned about performance-enhancing drugs than cannabis.

Julian Savulescu, co-director of the Wellcome Centre for Ethics and Humanities at the University of Oxford, U.K., told Newsweek: “It is a part of the futile U.S. war on drugs. There is no evidence that marijuana is performance-enhancing in sprinting. At one point that argument was made, but debunked.

“The problem is that ‘spirit of sport’ is defined in vague and unimplementable terms [like] fun, joy, teamwork, courage, etc.… and the real problem is that they don’t get good evidence of either performance-enhancing or lack of safety—they just guess.”

At the same time, Gregory Haff, professor of strength and conditioning at Edith Cowan University in Australia, told Newsweek he was “unaware of pressure from the U.S. government to dictate what is on the WADA code” and that WADA “was directly set up to be independent so no one country can dictate doping control policy.”

Newsweek has contacted WADA for comment.

What happens now?

The question now is whether marijuana’s place in WADA’s list of banned substances will be revised in light of the recent backlash.

Dr. Michael J. Joyner, an exercise physiologist and anesthesiologist who studies elite athletes at the Mayo Clinic in Minnesota, told Newsweek he thought it was “very likely” this would be the case.

Haff said there was still “much debate” about whether marijuana should remain on the list but added: “I do not believe social backlash should dictate what is on the list.”

Adair said there has been “sustained pressure” from athletes’ groups to remove cannabis from the list.

“Ultimately,” he said, “WADA will either liberalize its position or continue to ignore those pleas.”