The University of Virginia Medical Center, in Charlottesville, Va., typically uses 48 to 50 infusions of nitroglycerin a month. Since the restrictions were imposed in January, the hospital is limited to no more than 24 a month, said Dr. Robert E. O’Connor, the chairman of the emergency medicine department at the University of Virginia School of Medicine. He said the hospital’s stock was currently down to just four infusions, each of which would treat a patient for 18 to 20 hours. “It really limits how we can take care of patients,” he said.

Baxter said on Tuesday that it was increasing production of nitroglycerin and would go back to shipping the drug at the January levels by the end of this week. The F.D.A., which has taken the emergency step of hunting for overseas sources of the drug, also said that it expected the shortage to ease soon.

Capt. Valerie Jensen, associate director of the agency’s drug shortage program, said that it had identified another supplier outside the United States, and that she expected fresh supplies “in the coming weeks.”

“This is a critical drug,” she said in an interview. “We will have something in place.”

Captain Jensen added that a similar arrangement was underway with intravenous saline solution, which also has been in shortage — at times critically — for months. Baxter also manufactures intravenous saline.

But some expressed skepticism that Baxter would follow through with its promises.

“I sincerely hope that is true, but I don’t have a lot of faith that it will be because there is such a demand for this product,” said Erin Fox, a drug shortage expert at the University of Utah. She warned that Baxter and other drug manufacturers had frequently failed to meet their own deadlines, and she said that hospitals were struggling to make do, even at the previous supply levels. “We’ve seen from their estimated release dates for their saline products that those have been pushed off as well.”

Deborah Spak, a spokeswoman for Baxter, said the company had been forced to ration its shipments of nitroglycerin as it raced to order more raw materials and redirect operations to ramp up manufacturing of the drug. She said that fluctuations in allocations to hospitals were typical when a drug was in shortage.

“Baxter has been the one continuous supplier of nitroglycerin in the U.S. and the company is making every effort to meet the needs of customers,” she wrote in an email message, adding that the company was “hopeful that overall demand and supply will resume to a more predictable state within the next few months.”