Overwhelmed by demand for an experimental treatment for coronavirus, the drug maker Gilead abruptly shut down its emergency access program, leaving doctors and families scrambling for answers.

The company said it was setting up a broader access program that could try to help more people, but some said the transition is delaying remedies for very ill patients who have few options.

“We have heard zero. We know nothing,” said Genny Allard, the mother of Jack Allard, a 25-year-old New Jersey resident who is in a medically induced coma and on a ventilator at Hackensack Meridian Health JFK Medical Center in Edison, N.J. “I’m just, like, apoplectic at this point. I have a kid who is sick and the doctor wants to give him the next medicine that is supposed to help.”

The drug, remdesivir, is being studied in several large-scale clinical trials around the world, including a huge trial announced last week by the World Health Organization. But the results have not been reported yet, and it is still unclear whether the drug works against the coronavirus. It was studied to treat Ebola, but did not work well enough against that virus.

There is no known treatment against the coronavirus. Over the last week, President Trump has repeatedly referred to remdesivir and other drugs, like two long-used malaria drugs, as potential game-changers, despite pushback by top health officials like Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases. Dr. Fauci has corrected Mr. Trump in blunt ways during the daily White House briefings, saying that there is not enough scientific evidence to prove the drugs will work against the virus.

Critics of the president have complained that he is instilling false hope in a desperate public, as the disease spreads rapidly around the United States and elsewhere. Highly publicized talk of prospective cures has fueled demand for these potential treatments — threatening the longstanding supply of some of these drugs that are relied on regularly by people with lupus and other conditions.

As the virus spread to Europe and the United States, Gilead said emergency requests had “flooded an emergency treatment access system that was set up for very limited access to investigational medicines and never intended for use in response to a pandemic.”

Remdesivir has been given on a case-by-case basis to patients in China, Japan, Europe and the United States — where it was administered to the first coronavirus patient in the country, who was treated in Washington State in late January.

Six clinical trials of remdesivir are underway around the world, and early results from trials in China could be announced by the end of next month. “By the time we get to the end of April, we should have a preliminary idea of the safety and efficacy of this medicine against coronavirus,” said Daniel O’Day, Gilead’s chief executive, in a news conference earlier this month.

Gilead said it was switching to a broader program because it could not handle individual requests. “Due to overwhelming demand over the last several days, during this transition period we are unable to accept new individual compassionate use requests,” the company said on Sunday, with the exception of pregnant women and children under 18. “This approach will both accelerate access to remdesivir for severely ill patients and enable the collection of data from all participating patients,” the company said.

Gilead said it was processing “previously approved” requests but did not say what the status was of requests — like Mr. Allard’s — that had been submitted but not approved.

Drug makers have long run so-called “compassionate use” programs that allow patients to take unapproved, experimental drugs if they have no other options. The switch to what’s known as an “expanded access program” would allow the approvals to apply to larger groups instead of one individual at a time. These programs are often run as loose clinical trials, in which patients who did not qualify for the more stringent requirements of the trial can enroll. In addition to giving more patients access to the drug, it also allows the companies to collect data, including what dose was used, how sick the patient was and the extent that it worked. In compassionate-use programs, this type of information is usually not shared.

“You are trying to save somebody, rescue somebody. It’s a last-ditch throwing the kitchen sink at them, and you rarely get information back,” said Dr. Art Caplan, head of the division of medical ethics at the New York University Grossman School of Medicine. He said that he was giving unpaid advice to some companies pursuing treatments for coronavirus. The shift to a broader program will allow Gilead to learn about the drug. Without it, “you don’t learn what’s going on, so you’re sort of stuck.”

A Gilead spokeswoman said the company did not expect the transition to a new program would result in delays in treatment compared with how long patients were already waiting for the drug under the company’s now-ended compassionate-use program. Gilead would not provide details about the average turnaround time, but the spokeswoman said the company has been inundated with requests over the past week.

The company has said that it has a limited supply of remdesivir, but that it is increasing production. It is also drawing on a stockpile Gilead created for use in future pandemics after Ebola outbreaks in West Africa.

Mr. Allard, who lives in Metuchen, N.J. and works as an equity analyst for Bank of America in Manhattan, was admitted to the hospital on the night of March 16 after he had a high fever, back pain and was throwing up, his mother said. She described Mr. Allard, a former All-American lacrosse player at Bates College, as healthy and with no underlying conditions.

In New Jersey, officials announced 935 new positive cases on Monday, bringing the total to 2,844, including 27 deaths.

Mr. Allard was tested Tuesday for the coronavirus, but the sample was sent to Quest Diagnostics, and his mother said the hospital never received the results.

As the week went on, Mr. Allard’s condition rapidly declined, Ms. Allard said. He was placed in a medically induced coma and put on a ventilator. Ms. Allard said his doctors wanted to try remdesivir, but they needed to have a confirmed positive test showing he had the illness caused by the virus before they could seek the drug from Gilead. Finally, after the family made a flurry of calls to elected officials and others, the hospital tested him again on Saturday and received the results from another laboratory in less than six hours.

His results came back positive and that night — around 10:30 — the doctor submitted her request to Gilead. By the next morning, Ms. Allard said, she was hearing that Gilead was shifting to a new system, and news stories later on Sunday confirmed her fears.

His doctor has submitted another request under the new system, but Ms. Allard now fears his treatment will be delayed. Ms. Allard said the doctor had previously treated another patient with remdesivir and the turnaround time for approval had been 48 hours.

“I feel as if we are in regulatory limbo,” Ms. Allard said. “We were good, we were following the rules, he was queued up for what the rules were as of Saturday night and we woke up Sunday and everything changed.”