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 it 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 be 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.Election 2020 More