When President Trump promoted an experimental drug as a “cure” for Covid-19 in a video on Wednesday, it might have seemed that he was at it again: touting a questionable fix for a deadly pandemic, not unlike his earlier enthusiasm for the malaria drug hydroxychloroquine or even, at one point, disinfectant.

But the treatment that Mr. Trump extolled, which was administered last week after doctors diagnosed Covid-19, is not a fringe product. It’s a promising drug in the final stages of testing developed by a respected biotech company, Regeneron. Infectious disease experts have been closely following the treatment, as well as a similar product from Eli Lilly, in the hopes that the therapies could be a real advance in the fight against Covid-19.

Pharmaceutical companies often pay handsomely for celebrity endorsements, but this patient testimonial was like no other. It came from a polarizing president who, just weeks away from an election, and having found himself and his White House at the center of an outbreak, is eager to show that his administration is doing something about a pandemic that has killed more than 212,000 Americans.

Although he couldn’t possibly have known whether Regeneron’s treatment had helped him — or even if he was out of the woods yet — Mr. Trump sang its praises in the video, calling it “unbelievable” and suggesting it was only moments away from being authorized for widespread use. In doing so, Mr. Trump reminded his critics of the many times — from reopening schools to authorizing hydroxychloroquine and blood plasma — over the past nine months that he has inserted politics into the decisions of independent health agencies.

Regeneron, which filed an application with regulators within hours of the president’s video, must now shepherd its antibody treatment through a politically fraught approval process, where the president’s over-the-top endorsement has likely raised the profile of its product, but could also sow suspicion about whether it works.

“I don’t see how it is going to end up being good for a pharma company,” said Ronny Gal, a pharmaceutical analyst for the Wall Street firm Bernstein. “Once you become a political opinion, that’s not great.”

Already, Regeneron is fielding messy questions about how its treatment was tested using cells originally derived from an aborted fetus — a line of research that Mr. Trump has opposed — and the president’s relationship with Regeneron’s chief executive.

Mr. Trump has further complicated the potential rollout of these treatments by pledging — first on Wednesday and again in another video Thursday — that the drugs would be free of charge and would soon be available in hundreds of thousands of doses.

But Regeneron said it would only initially have enough doses for 50,000 patients, with the plan to have enough for about 300,000 people by the end of the year. Regeneron has received more than $500 million in federal funding to develop and manufacture the treatment, and through that deal, the company has said it will make the products available at no cost to Americans.

Still, that’s a small number, given the scale of the outbreak in the United States and the fact that the treatment is believed to work best soon after infection. On Wednesday alone, more than 50,000 Americans tested positive for the virus.

“This is like a massive direct-to-consumer advertising campaign for a product where we have scarce supply and limited capacity to treat, which is a nightmare for companies in the industry,” said Geoffrey Porges, an analyst for SVB Leerink, an investment bank in Boston.

There is no way to know if Regeneron’s antibodies have helped Mr. Trump. The president was given several drugs at Walter Reed National Military Medical Center, including the antiviral remdesivir and the steroid dexamethasone, which have been proven to help patients with Covid-19.

Mr. Trump has said he is feeling better, but his doctors have provided sparse and conflicting details about his health, and he has only just entered the second week of the disease, when some patients take a turn for the worse.

Dr. Mark Mulligan, director of the N.Y.U. Langone Vaccine Center, who is involved in studies of both Regeneron’s and Eli Lilly’s antibody products, said the president’s claim that he was cured seemed premature — though not impossible.

“We know sometimes people will get better and then worse,” Dr. Mulligan said. “I would want to reserve judgment and hope he’s on a good trajectory.”

The only way to know whether a treatment works is to test it in large groups of patients, comparing those who got the drug to those who got a placebo.

Monoclonal antibodies, the treatments developed by Regeneron and Eli Lilly, are believed to work by giving patients powerful antibodies that help fight the virus. Like Regeneron, Eli Lilly has also recently asked the Food and Drug Administration for emergency authorization of its treatment.

Although the companies’ trials are not complete and they have not published their findings in medical journals, early data has shown promise. Eli Lilly is testing two treatments: one that uses a single antibody and another that uses two. Both have shown evidence that they reduced the rate of hospitalization in patients who got the treatments soon after they tested positive.

Regeneron recently released data showing that its drug, a cocktail of two antibodies, appeared to help the body clear the virus when it was given early in the disease.

“We feel like the early indications of antiviral activity and potential impact on the clinical course of disease is very promising,” Dr. Janet Woodcock, who is overseeing the federal effort to speed coronavirus treatments to market, said on Friday in a call with reporters. She said the emergency use application was now in the hands of the F.D.A.

While they wait for the government’s decision, Eli Lilly and Regeneron now face the challenge of preserving their credibility even as the president continues to make unfounded and exaggerated claims.

“When he comes out and says, ‘Great news, I’m cured,’ it can only be seen through the political lens of his re-election,” said Arthur L. Caplan, a professor of medical ethics at the N.Y.U. Grossman School of Medicine.

Now Regeneron will need to prepare for a potential surge in demand. “They are in a thicket of ethical difficulties, awaiting what I think will be a pretty big onslaught of further requests,” Dr. Caplan said.

All of the attention has raised the profile of Regeneron, a lesser-known pharmaceutical company based in Westchester County that sells the eye drug Eylea and the asthma drug Dupixent.

Since the president received the experimental treatment last Friday, a company spokeswoman has said they have seen an uptick in requests for the drug outside of clinical trials — through what’s known as “compassionate use” — although she did not provide details. Enrollment in the trials has also increased, she said: Over the past two weeks, the company has enrolled 500 participants into its trials, for a total of 2,500 people.

But the scrutiny has not all been positive. Some have raised questions of fairness, such as why the president should have been given access to a not-yet-available drug when tens of thousands of Americans are sickened with the virus every day.

“I can’t offer it to my patients,” said Dr. Matthew G. Heinz, a hospitalist at a large medical center in Tucson, Ariz., who acknowledged that the treatment’s benefits are still not proven. “That’s the most frustrating thing to me as a physician.”

Others have questioned the president’s relationship to Dr. Leonard S. Schleifer, the billionaire co-founder and chief executive of Regeneron, who has been a member of Mr. Trump’s golf club in Westchester County. The two have known each other casually for years, and Dr. Schleifer has told associates that Mr. Trump calls him to ask about the status of the coronavirus treatment.

But Dr. Schleifer is also a frequent donor to Democratic candidates, and his son, Adam Schleifer, ran unsuccessfully in New York’s Democratic primary this spring for the seat vacated by Rep. Nita Lowey.

The company also had to field questions this week about its use of cells derived from fetal tissue to test the antibody cocktail. The cell line, developed decades ago, has been used to test many other drugs, including remdesivir and some of the coronavirus vaccines in trials.

The juxtaposition of Mr. Trump’s boosterism with his opposition to the use of fetal tissue in scientific research struck his critics as rich with hypocrisy. But the president’s enthusiastic peddling of Regeneron’s antibodies may at least have brought public attention to treatments that could eventually pan out.

“The fact that he was given one of these therapies has increased the awareness of them,” Dr. Mulligan said. “I think in a way it’s a good thing. We need some successes.”

Gina Kolata and Noah Weiland contributed reporting.