Vladimir Zelenko, a self-described “simple country doctor” from upstate New York who rocketed to prominence in the early days of the COVID-19 pandemic when his controversial treatment for the coronavirus gained White House support, died Thursday in Dallas. He was 48.

His wife, Rinat, said he died of lung cancer at a hospital where he was receiving treatment.

Until early 2020, Zelenko, who was also known by his Hebrew name, Zev, spent his days caring for patients in and around Kiryas Joel, a village of about 35,000 Hasidic Jews roughly an hour northwest of New York City.

Like many health care providers, he scrambled when the coronavirus began to appear in his community. Within weeks, he had landed on what he insisted was an effective cure: a three-drug cocktail of the antimalarial drug hydroxychloroquine, the antibiotic azithromycin and zinc sulfate.

He was not the first physician to promote hydroxychloroquine. But he began to draw national attention on March 21, 2020 — two days after President Donald Trump first mentioned the drug in a press briefing — when Zelenko posted a video to YouTube and Facebook in which he claimed a 100% success rate with the treatment. He implored Trump to adopt it.

A day later, Mark Meadows, Trump’s chief of staff, reached out to Zelenko for more information. So did talk-show bookers. Over the next week, Zelenko made the rounds on conservative media, speaking on podcasts hosted by Steve Bannon and Rudy Giuliani. Sean Hannity of Fox News touted his research during an interview with Vice President Mike Pence.

“At the time, it was a brand-new finding, and I viewed it like a commander in the battlefield,” Zelenko told The New York Times. “I realized I needed to speak to the five-star general.”

On March 28, the Food and Drug Administration granted emergency authorization to doctors to prescribe hydroxychloroquine and another antimalarial drug, chloroquine, to treat COVID-19. Trump called the treatment “very effective” and possibly “the biggest game changer in the history of medicine.”

But, as fellow medical professionals began to point out, Zelenko had only his own anecdotal evidence to support his case, and what little research had been done painted a mixed picture.

Still, he became something of a folk hero on the right, someone who offered not just hope amid the pandemic but an alternative to the medical establishment and Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, who insisted that months of research would be needed to find an effective treatment.

Zelenko continued to text and speak with Meadows, Giuliani and several members of Congress. But he clashed with leaders in Kiryas Joel, who said his talk of treating hundreds of COVID-19 patients gave the impression that the community was overwhelmed by COVID-19, potentially stoking antisemitism.

Over the next few months, researchers cast further doubt on the efficacy of hydroxychloroquine. A study published in The New England Journal of Medicine found no benefit from the treatment, and other studies highlighted a risk of dangerous heart arrhythmias in some patients.

Those results and others led the FDA to revoke its emergency authorization on June 15, 2020.

A quiet, unassuming man, Zelenko seemed unprepared for the attention he received, which included harassing phone calls and even death threats. In May 2020, a federal prosecutor opened an investigation into whether he had falsely claimed FDA approval for his research.

That same month, Zelenko announced in a video that he was closing his practice and leaving the Kiryas Joel community. He accused several of its leaders of instigating a campaign against him.

After the FDA rescinded its approval of hydroxychloroquine as a COVID-19 treatment, he founded a company, Zelenko Labs, to promote other nonconventional treatments for the disease, including vitamins and quercetin, an anti-inflammatory drug.

And although he claimed to be apolitical, he embraced the image of a victim of the establishment. He founded a nonprofit, the Zelenko Freedom Foundation, to press his case. In December 2020, Twitter suspended his account, stating that it had violated standards prohibiting “platform manipulation and spam.”

Zelenko was born Nov. 27, 1973, in Kyiv, Ukraine, and immigrated to the United States with his family when he was 3, settling in the Sheepshead Bay section of New York City’s borough of Brooklyn.

His father, Alex, drove a taxi, and his mother, Larisa (Portnoy) Zelenko, worked in a fur factory and later, after studying computer programming, for Morgan Stanley.

In a memoir, “Metamorphosis” (2018), Zelenko wrote that he grew up nonreligious and entered Hofstra University as an avowed atheist.

“I enjoyed debating with people and proving to them that G-d did not exist,” he wrote. “I studied philosophy and was drawn to nihilistic thinkers such as Sartre and Nietzsche.”

But after a trip to Israel, he began to change his mind. He gravitated toward Orthodox Judaism and, in particular, the Chabad-Lubavitch movement.

He graduated from Hofstra in 1995 with a degree in chemistry, and he received his medical degree from the State University of New York at Buffalo in 2000. After returning to Brooklyn for his residency, he moved to Monroe, a town neighboring Kiryas Joel, in 2004.

Zelenko spent three years working for Ezras Choilim, a medical center in Monroe, and advising the local Hatzolah ambulance service. He opened his own practice in 2007, with offices in Monroe and Monsey, another upstate town with a large Orthodox Jewish population.

In 2018, doctors found a rare form of cancer in his chest and, in hopes of treating it, removed his right lung.

Zelenko’s first marriage ended in divorce. Along with his second wife, he is survived by their two children, Shira and Liba; six children from his first marriage, Levi Yitzchok, Esther Tova, Eta Devorah, Nochum Dovid, Shmuel Nosson Yaakov and Menachem Mendel; his parents; and a brother, Ephraim.

This article originally appeared in The New York Times.