Learn about the history and usage of Clenbuterol in cattle and the effects it can have on professional athletes. (3:36)

Read the Spanish-language version of this story here.

MEXICO CITY - SITTING ATOP A SMALL, wooden fence, the rancher surveys his surroundings. The cows gather in the shade, escaping the hot Central Mexico sun. They feed on a mixture of cornhusks, hay and other organic scraps.

The rancher is coy as he describes his work. These cows are "clean" he says, but hypothetically, he adds with a wink, they can all be "dirty" -- thus his request for anonymity. Inside the barn, he opens an empty drawer and a hidden compartment reveals a bag filled with a white powder. "Here's the cocaine," he says cracking a smile.

The white powder is actually clenbuterol, a potent drug that, according to the U.S. Drug Enforcement Agency, is often abused by bodybuilders and other athletes for its ability to increase lean muscle mass and reduce body fat. It is illegal for human use in most of the world, including Mexico. However, it is used commonly on farms here, administered to livestock to produce the same physical effects.

Originally developed as medication for people with breathing disorders, it became obsolete due to its potentially dangerous side effects, which can include heart palpitations, muscle tremors and anxiety. Though clenbuterol is banned by the World Anti-Doping Agency, a number of athletes - from professional cyclists to soccer stars to elite boxers, such as Canelo Alvarez - have made headlines, not just for testing positive, but also for the accompanying defense.

It was the tainted meat.

Some experts say claiming to eat contaminated beef is a convenient excuse.

There is no clear answer as to how much of Mexico's beef is tainted. Experts, including medical researchers, veterinarians and former government employees, estimate that number to be anywhere from 40 to 80 percent. Alejandro Cegarra for ESPN

AT A PACKED news conference in April, Canelo Álvarez sat behind the microphone, shoulders hunched and arms crossed. The multiple-time boxing world champion looked defeated.

"I'm very surprised with what's happened," he said in Spanish to the room full of reporters. "I'm sorry that this brings doubts to my integrity as the clean athlete I've always been, and I'd like to offer an apology."

Canelo was in Los Angeles to withdraw from the much-anticipated May 5 rematch against rival Gennady Golovkin (the two will fight this weekend in Las Vegas). The Mexican boxer had failed two drug tests and faced the possibility of a one-year suspension. He claimed the positive test was the result of his eating beef contaminated with clenbuterol in his home country.

Clenbuterol's appearance on the banned substance list is enough for any positive test to land an athlete in trouble. In a statement delivered to the Nevada Athletic Commission and provided to ESPN, Álvarez said he had eaten beef in Mexico on at least six occasions ahead of his positive test in February. That included tacos at a party and tortas from a street vendor.

"This was my mistake for not reading up on the risks, not researching more, more on the subject, on what's going on with the beef in Mexico," Álvarez said to ESPN's Ramona Shelburne through a translator in June.

Some experts say the drug can provide distinct advantages for athletes such as boxers.

"Aside from the fat-burning and muscle-building, [clenbuterol] opens up airways and facilitates breathing, it accelerates the metabolic rate and stimulates the central nervous system, so it helps for a faster reaction time," said Victor Conte, the founder and former president of BALCO, the laboratory at the center of controversy for its role in providing steroids to professional athletes.

Despite WADA's ban and Conte's assertion to the substance's effectiveness, not everyone agrees that clenbuterol is conclusively beneficial to athletic performance.

"It has the potential to offer those benefits," said Dr. Matthew Fedoruk, chief science director at U.S. Anti-Doping Agency, a signatory to WADA. "The problem is that human studies haven't been done to support this. From an athlete's perspective, it could possibly have those effects [on humans], but there's a lot of debate out there as to whether clenbuterol is really anabolic in humans at all. Some say more through anecdotal evidence that it builds muscle, others are saying that it doesn't, so the jury's kind of out there in respect to human beings."

After testing positive for clenbuterol, Canelo Álvarez had to back out of a May 5 fight against Gennady Golovkin. Álvarez was able to clear his name, and the rematch is Sept. 15. Kevork Djansezian/Getty Images

American sports leagues have also contended with similar claims from their athletes. In 2015, then-Houston Texans offensive tackle Duane Brown faced a 10-game suspension from the NFL after testing positive for clenbuterol. He had just returned from a vacation in Cabo San Lucas, Mexico, and blamed the result on tainted beef. To fight the charges, Brown provided receipts that indicated he had eaten beef in the Mexican resort city. After a months-long process, the suspension was revoked.

Stemming from Brown's case, the National Football League Players Association issued a warning for players to avoid consuming beef when traveling to Mexico and China, another high-risk country on WADA's radar. Since 2016, one regular-season game has been played at the Estadio Azteca in Mexico City each year, with the Los Angeles Rams and Kansas City Chiefs scheduled to play there on Nov. 19. Subsequently, the league asked the NFL Mexico office director, Arturo Olivé, to exert greater control over what players eat in the country

"We took the necessary steps: We've been in contact with the hotels [to avoid contamination]," said Olivé in 2016, before the Texans faced the Oakland Raiders at the Estadio Azteca.

MLB and the NBA have also taken measures to protect their athletes when games are played in Mexico. An NBA spokesperson told ESPN the league provides teams with meals at hotels, makes recommendations about local restaurants and "advises all personnel to exercise caution in consuming certain foods when traveling abroad."

The recommendation not to eat Mexican meat prompted Sarah Colonna, the comedian and wife of former Seattle Seahawks punter Jon Ryan, to joke on the restriction when the pair vacationed in the country last July.

Question: since @JonRyan9 can't eat beef in Mexico due to possible steroids in it (SERIOUSLY), as a good wife should I :



A) order a hamburger and eat it right in front of him



B) order 2 hamburgers and eat them both right in front of him — Sarah Colonna (@sarahcolonna) July 7, 2018

Though the knowledge of Mexico's meat situation might register as a minor nuisance for visitors from outside the country, for athletes living, working and training in the country, the situation can prompt bigger issues. "'I'm trying to get used to not eating beef. Because most of the time I'm in Mexico, and I've stopped eating beef," Álvarez said.

In Erik Morales' case, a positive test left a lasting effect beyond his athletic career. The former Mexican professional boxer who won world championships in four different weight classes, was flagged for clenbuterol in 2012, before his fight with Danny Garcia in Brooklyn. "If it happens to one person, I understand. You might have doubts," said Morales. "But think about how many Mexican athletes this has happened to, you would have to be crazy to ignore that. It's no coincidence." Garcia agreed to have the fight go on anyway, and proceeded to beat Morales. Morales was subsequently suspended for two years by the USADA, retiring shortly after.

Now a Congressman for Mexico from his home state of Baja California, Morales says he is pushing legislation to further regulate the meat industry and tackle the problem. "We shouldn't have to worry about this. You have to be able to go out and not be worried about your livelihood just because you ate a steak or a taco," said Morales.

Ranchers in Mexico say beef buyers are looking for cattle raised on clenbuterol because the cuts are leaner and there's more product to sell. Alejandro Cegarra for ESPN

THE RANCHER MIMICS spooning some of the powder out of the bag and sprinkling it onto the cows' food, demonstrating how to administer clenbuterol. "It's really a small amount per animal," he says, "but I'll have several cows eating at a time, so I have to compensate for that.

You should see them afterward, they're running around all over the place like crazy."

He pauses, waiting for his encore joke to register.

Though clenbuterol is banned for use on humans and as a cattle enhancer, it is still legal for veterinarians to prescribe in specific circumstances in Mexico. In these cases, it is generally used on horses with respiratory problems and as a muscle relaxant for pregnant cows to alleviate the birthing process. It is also available for purchase on the internet, largely from overseas distributors. Both of these methods have the potential to leave a paper trail, so some ranchers prefer a third option: black market dealers.

"People will come to your farm and sell it to you directly," the rancher says.

Cows consuming the substance grow significantly larger than their naturally fed counterparts and, as a result, can be in higher demand when it comes time to sell. In some cases, the buyers - who distribute the beef to restaurants, hotels and street vendors all over Mexico - specifically ask for cattle raised on the drug because the cuts are leaner and there's more product to sell. If the ranchers do not comply, they say, the buyers simply move on to someone who will fulfill their request.

"That pushes ranchers into a corner. If their cows are not dirty, there's a good chance they can't sell them. And if you can't sell, you can't provide for your family," said Octavio Campuzano, a veterinarian who specializes in bovine medicine and surgery at the National Autonomous University of Mexico, the country's largest public university.

Dr. Campuzano notes the need to get product out the door for ranchers can lead to dangerous practices, like higher dosing. "To achieve the desired result, you need a tiny, microscopic amount per cow to develop the muscle." For a group of cows eating from a trough, a few spoonfuls of the drug is enough. "But these people are not experts, and they might think [bigger quantities] will lead to faster, better results," he said.

Higher doses can result in higher concentrations of the drug in the meat, and for a consumer to potentially fall ill.

In 2012 in the resort city of Cancun, the Mexican government confirmed 90 cases of people who displayed symptoms that included vomiting, diarrhea and tachycardia, consistent with clenbuterol poisoning. Some taco stands were shuttered.

"The highest concentration of clenbuterol will always be in liver meat," Campuzano says. "But any cut of meat can be dangerous if the cow was dosed with a high enough quantity." In response to cattle doping, Mexico's Congress approved an eight-year prison sentence for those caught producing or consciously distributing tainted meat.

WADA took notice and released several warnings about contaminated beef in Mexico and China. In 2011, the agency stated, "It has been shown that Mexico and China have a serious problem with meat contaminated with the prohibited substance clenbuterol, and WADA's message to athletes competing in these countries remains the same: eat only in restaurants and cafeterias that have been approved by your federation and/or event organizer."

Álvarez named Mexican chain restaurant Sonora Grill (Mexico City location pictured) as one restaurant where he ate beef ahead of his February drug tests. Alejandro Cegarra for ESPN

ATHLETES WHO ALLEGE meat contamination as the cause for clenbuterol in their system often find themselves in an uphill battle.

Edgar Dueñas was one of five players on Mexico's national soccer team to be suspended during the 2011 CONCACAF Gold Cup following a positive clenbuterol test. "When they gave us the news, we were all shocked," said Dueñas, a defender who played in Mexico's top division, Liga MX. "It was completely unexpected."

Ahead of the U.S.-hosted tournament, the team was on a strict diet imposed by the Mexican Football Federation, with the beef coming from the federation's approved supplier. Dueñas recalls the doubts cast on his integrity at the time. "What they didn't understand is that we had taken the tests before leaving Mexico," he said.

Upon the release of the initial results, WADA presented an appeal to the Court of Arbitration for Sport to have the players suspended at the club level, as well. "There was a lot of uncertainty, and they were pushing to suspend us for a long time," Dueñas said.

To support their claim, Dueñas and his teammates requested a second test from the initial samples. This is known as a "B" sample. They submitted hair follicles, which can store substances up to 90 days, to determine whether traces of the drug were present. While a single dosage can show up on a urine test, a positive result stemming from hair follicles can indicate repeated consumption. That test came back negative. Concurrently, there were widespread positive results for players from multiple countries at the 2011 FIFA Under-17 World Cup held in Mexico. WADA withdrew the appeal.

But the low amount of human research around clenbuterol, coupled with what some would deem a built-in alibi, creates a scenario in which athletes attempt to game the system in Mexico and abroad, according to experts. "Well, that's the dog-ate-my-homework excuse," said Conte.

Spanish cyclist Alberto Contador is the best example of this. A urine sample taken during the 2010 Tour de France revealed traces of clenbuterol. The two-time Tour champ pointed to a seven-pound cut of tainted veal tenderloin he purchased in Spain. But his claim did not hold up. Authorities traced the source of the meat and, after inspecting the farm from which it originated, found no evidence cows were being doped with clenbuterol. In 2012, he was found guilty by the Court of Arbitration for Sport, ending a two-year proceeding in which Contador was initially punished, then cleared and subsequently retried and penalized. He was also stripped of 13 victories.

At the request of the Nevada State Athletic Commission, Canelo Álvarez submitted to the hair follicle test on March 29, a month after his original positive urine test. The hair follicle sample came back negative for clenbuterol. "They found traces of clenbuterol in his system and if he was using it (to dope) there would probably still be enough in his system that it would show up in his hair," Golden Boy Promotions CEO Oscar De La Hoya told ESPN's Dan Rafael in April.

To avoid potential cheating from those like Contador and others who blame contaminated meat, Conte supports hair follicle testing to better judge whether any positive test was accidental. "In Canelo's case, they took a hair from the back of his neck, and there was no clenbuterol," he said. "So, this supports Canelo's explanation."

Canelo's rematch with Golovkin was eventually rescheduled for Sept. 15.

According to Octavio Campuzano, a veterinarian at the National Autonomous University of Mexico, it's impossible to regulate the practice of clenbuterol doping. "The government doesn't have the manpower to physically inspect all of the meat," he says. Alejandro Cegarra for ESPN

DARK CLOUDS roll over the ranch, signaling the coming afternoon rains that define summers in this part of Mexico. The rancher is asked what he would do if he were caught administering clenbuterol to his cattle. "I don't know what I'd do, really," he says. "It's scary. You hear stories about people who have been punished, taken to jail."

But those stories are rare. Campuzano says it's impossible to regulate the practice of clenbuterol doping. The Secretariat of Agriculture, Livestock, Rural Development, Fisheries and Food -- the Mexican government agency that regulates the production, sale and distribution of cow beef -- launched a program in 2015 to eradicate the use of clenbuterol and other substances by providing by-the-book ranchers with a seal of quality, guaranteeing consumers contamination-free meat.

However, the measure relies on ranchers signing up voluntarily. Campuzano and other experts have deemed it a failure.

"The government doesn't have the manpower to physically inspect all of the meat," Campuzano says. "And when they do get people in there from the government to inspect, there's also the issue of corruption."

Back at the ranch, the question of conscience arises: the cause and effect of feeding clenbuterol to his cattle and exposing consumers to the potentially damaging drug. How does the rancher justify his actions?

"Listen, if I didn't do it, it wouldn't make a difference," he says. "Somebody else would, and I'd starve."