Simos Simeonidis, a senior biotechnology analyst at the New York investment bank Rodman & Renshaw, who wrote a report on MannKind, said he expected its system to be available next year, if the F.D.A. approved. (Dr. Simeonidis has no stock in MannKind, but Rodman & Renshaw has provided investment banking services for it.)

Image An insulin inhaler from the MannKind Corporation is awaiting F.D.A. marketing approval. Credit... David A. Kramer

Leonid Poretsky, chief of endocrinology at Beth Israel Medical Center in New York and director of the Gerald J. Friedman Diabetes Institute there, said that the MannKind system will face many problems even if it is approved.

“Injections today are essentially painless,” he said of the short, thin needles that are commonly used to inject insulin. And you don’t necesssarily “have to draw from a bottle into a syringe. Injections work so well that the advantages of a new route like this are unclear.”

Dr. Poretsky was also concerned about using the lungs to transport drugs. “It’s possible for people to stay on insulin for decades,” he said. “The whole issue of exposing the lungs to insulin for a long period of time has to be examined carefully.”

Dr. Gerald Bernstein, a New York-based endocrinologist who is a former president of the American Diabetes Association, agreed that the long-term use of inhalable insulin might carry risks for some patients. Dr Bernstein is vice president of the Generex Biotechnology Corporation, which is developing an insulin delivered through the lining of the mouth.

“It’s counterintuitive to use the fragile cells of the alveoli,” the tiny air sacs within the lungs, “to get insulin to the bloodstream,” he said. “The lungs were developed to transport gases, not proteins.”

Mr. Pfeffer of MannKind said that the company’s clinical data included no signs of damage to lungs.

DR. DAVID M. NATHAN, a professor of medicine at Harvard Medical School and director of the Massachusetts General Hospital Diabetes Center, said that even if safety issues were addressed, there could be other long-term problems with Afresa. He questioned whether MannKind’s inhalable product could achieve the same level of blood sugar control as that obtained with injections or insulin pumps.