The UFC and United States Anti-Doping Agency (USADA) have made significant revisions to the promotion's anti-doping policy in an effort to address concerns over an alarming rate of cases involving contaminated supplements.

The two major revisions, which the UFC announced on Monday, include an adoption of a "UFC prohibited list," which sets threshold limits on what constitutes a positive drug test for several banned substances, as well as a list of "certified supplements" that offer immunity to athletes in the event they are found to be contaminated.

"The policy needs to be a living, breathing document that's progressive and allows us to pivot our stance, based on what the science supports," UFC chief business officer Hunter Campbell told ESPN. "Testing measures have gotten so good and the types of contaminants are changing constantly, you're seeing a rise in these types of cases."

Since the UFC partnered with USADA and launched its year-round drug testing program in 2015, a significant percentage of the cases involving failed drug tests have ultimately been linked to the ingestion of contaminated legal supplements, according to UFC senior vice president of athlete health and performance Jeff Novitzky.

Essentially, the UFC and USADA have found that as drug testing technology advances -- and is capable of detecting extremely trace amounts of banned substances -- it is resulting in the punishment of athletes who have unknowingly consumed contaminated supplements and received no performance-enhancing benefits from them.

One of the highest-profile cases occurred last month, involving UFC star Nate Diaz. Diaz submitted a drug test that showed a very trace amount of a banned selective androgen receptor modulator (SARM), which entered his system through a contaminated organic, vegan multivitamin. Diaz was cleared of any wrongdoing and still fought Jorge Masvidal at UFC 244 on Nov. 2, but had that instance occurred during the first year of the program, the fight would have almost assuredly been canceled.

"We're thrilled with these changes and hope it becomes the model for all sports organizations," USADA CEO Travis Tygart told ESPN. "We believe it looks at the advances in technology coupled with the marketplace for supplements. We've had a number of cases both in our Olympic program and UFC program where we demonstrated trace levels of substances coming from totally innocent sources. We've seen a gummy multivitamin that had a SARM in it."

The UFC's new prohibited list still follows most of the World Anti-Doping Agency (WADA) code, but establishes thresholds for certain substances, including SARMS and diuretics. The UFC and USADA relied on testimony from multiple anti-doping experts to set these levels. The promotion also identified five independent supplement certifiers. Should an athlete fail a test due to a contaminated supplement that's been certified by one of those agencies, that athlete would not receive any sanction, whereas that was previously not the case.

"If the athletes adhere to this, if they only use these certified supplements, I believe it will virtually eliminate the contaminated supplement issue," Novitzky said.

According to Novitzky, the new policy goes into effect immediately and would apply to any open cases still pending.

This is the third, and arguably most significant, revision of the UFC's anti-doping policy since 2015. The UFC and USADA extended its partnership for multiple years in December 2018. In 2019 thus far, USADA has administered 3,756 tests involving 696 UFC athletes, according to online records.

According to Campbell, the process of these latest revisions began in earnest last year, around the development of another high-profile case involving UFC light heavyweight champion Jon Jones. Jones has submitted multiple drug tests that have produced atypical findings for a trace amount of a steroid metabolite. Experts have found, however, the atypical findings are likely the result of a "pulsing" effect of that metabolite, which means it could remain in Jones' system indefinitely, even if he never re-ingests the parent substance.

"Jon Jones was a moment where we all sat down and said, 'We need to take a look at what the science is, as it pertains to the prohibited list,'" Campbell said. "And I want to give Jon some credit because he took criticism like no one else has during that case, but everything that occurred in that case turned out to be true and helped result in where the policy is today."