The Food and Drug Administration on Friday approved a form of insulin that is inhaled rather than injected, providing a new option for millions of Americans with diabetes.

The approval of the drug, Afrezza, is a milestone in a long and expensive journey for its developer, MannKind Corporation, and for its chief executive, Alfred E. Mann, an 88-year-old billionaire who spent a big part of his fortune sustaining the company through various setbacks.

“Today’s F.D.A. action validates the years of clinical research and commitment that powered the development of this unique therapy,” Mr. Mann said in a statement Friday.

But MannKind still faces the challenge of selling the drug, which was approved for adults with either Type 1 or Type 2 diabetes.