Now city authorities are facing a backlash. A U.S.-based academic site that had posted their paper, SocArXiv, withdrew it last Friday, charging it was “promoting an unproved medical treatment in the midst of a global pandemic.” The site accused city officials of bad science and unethical behavior — in effect, of using citizens like rats in a giant laboratory experiment, without their consent.

The decision has detonated a storm on social media. Opposition politicians are demanding an investigation.

What makes the scandal remarkable isn’t just the scale of Mexico City’s program — nearly 200,000 kits with ivermectin were distributed — but who was advocating it. Unlike in the United States, where ivermectin has been promoted by conservative commentators (and star podcaster Joe Rogan), the drug was championed in Mexico by leftist intellectuals in top government jobs.

Mexico City’s government has boasted of confronting the coronavirus with science-driven policies including widespread testing and vaccination. Mayor Claudia Sheinbaum holds a doctorate in environmental engineering. Yet even as Mexico’s federal government cautioned against using ivermectin to treat the illness, desperate city leaders were lured by a drug that has dazzled politicians from Alaska to India.

The Mexico City program began in December 2020. Covid-19 cases were spiking, and hospitals were jammed. An alarmed city government provided medical kits to patients who tested positive for the coronavirus and had mild to moderate symptoms. Each contained four tablets of ivermectin.

After handing out 83,000 kits, the government crunched the numbers. It reported a decline of at least 52 percent in hospital admissions among those who had received the kits, compared to others previously infected. “It’s GREAT news to be able to validate our policy,” tweeted city official Jose Merino.

At first, the government’s study appeared destined to fade into obscurity, like many of papers submitted to SocArXiv. “I wasn’t even aware of this,” said Philip N. Cohen, the sociologist at the University of Maryland who runs the online archive, which features more than 8,000 research papers. “Until it blew up.”

SocArXiv, founded in 2016, provides a forum for social scientists to share their research before it’s been peer-reviewed. Such sites have become more popular as academics try to post their latest findings — and get quick feedback from colleagues — in the fast-moving era of the coronavirus.

In late 2021, Cohen was startled to see that the top paper that year had been the study “Ivermectin and the odds of hospitalization due to COVID-19” based on the Mexico City program. It had been downloaded more than 10,000 times.

Then Cohen began to hear from academics — including Juan Pablo Pardo-Guerra, a sociologist at the University of California at San Diego, who wrote a widely shared Twitter thread assailing Mexico City’s work. “This paper is comparable in its ethical dimensions to the infamous Tuskegee study,” he wrote. The Tuskegee experiment was a 40-year study in which the U.S. Public Health Service observed the effects of syphilis in hundreds of Black men without informing them of their diagnosis or treating the disease.

Cohen and his colleagues examined the Mexico paper and concluded that it had multiple problems.

One was that the medical kits included not just ivermectin but paracetamol, aspirin and oximeters. It wasn’t clear which of the items may have improved patients’ health, Cohen said, and the subjects of the study weren’t chosen randomly, as in a clinical trial. In addition, the city government hadn’t declared its conflict of interest — in other words, that it would benefit if the study portrayed the program as a success. And the city was mass-distributing a medication that international authorities, including the World Health Organization, said should be used to treat covid-19 only in clinical trials.

What was most worrisome to Cohen is that the 10,000 downloads were probably not by scientists pondering the program’s merits. “We thought the paper was being used to spread misinformation,” Cohen said.

Mexico City authorities have denied that the program was an unethical experiment.

The health secretary, Oliva López Arellano, said in the decision to use ivermectin was made at “a different moment” in the pandemic, before vaccines were widely available. Some other cities around the world were also offering the medication to citizens sickened in the pandemic, she said. While she acknowledged the controversy, “it was more in terms of whether it was useful” in treating covid-19, she said, and not whether small doses could cause harm.

The real problem wasn’t such limited doses, she said, but a trend in the United States and elsewhere of citizens poisoning themselves with large amounts of ivermectin, or even using a version intended for animals. “What we want to signal is, all the kits were always given out by health professionals, with safe doses,” she said.

It’s highly unusual for a government body to distribute a medication that even the country’s own drug regulatory body hasn’t authorized to treat an illness. Mexico’s senior health officials — including the coronavirus czar, Hugo López-Gatell — have consistently warned citizens not to use ivermectin for covid-19.

Merino, the lead author of the Mexico City study, accused Cohen of being “colonialist and authoritarian” in withdrawing the article. Merino, a political scientist who heads the city’s Digital Agency for Public Innovation, acknowledged that the ivermectin study wasn’t a clinical trial, but rather an observational study, in which researchers analyze the effect of an intervention without controlling who is affected.

“It’s very obvious that in the United States the simple mention of ivermectin triggers a political and media frenzy,” he wrote in a letter signed by most of the study’s co-authors.

Mexico City stopped distributing ivermectin in September and now bases its covid-19 strategy on vaccines.

Sheinbaum, a protege of President Andrés Manuel López Obrador, is widely seen as a top candidate to succeed him when his term ends in 2024. From the onset of the pandemic, her government has tried to distance itself from López Obrador’s efforts to play down the coronavirus, urging greater mask usage and coronavirus testing,

Still, the densely packed city has been one of the world’s hardest-hit capitals during the pandemic. According to government estimates, it has suffered more than 86,000 covid-19 deaths.