Dr. Gottlieb, who visited F.D.A. staff in Puerto Rico last week, told the House Energy and Commerce Committee’s subcommittee on Health: “We have a list of about 40 drugs that we’re very concerned about. It reflects maybe about 10 firms.”

Thirteen of the drugs, Dr. Gottlieb said, are “sole-source,” meaning the product is made only by one company. Those include H.I.V. medications, injectable drugs and sophisticated medical devices, although he did not name the products. The biggest problem, he said, was not damage to the factories, but the instability of the electric supply. Manufacturers are worried that a long-term lack of connection to a major power grid could jeopardize their products, and are also wary of relying on the more limited electrical grids that the territory is likely to activate as a first step to restoring power.

One of the drugs F.D.A. officials said they were concerned about was methotrexate, which treats childhood leukemia and other diseases. It has been scarce, off and on, for several years. Mylan makes the product in Puerto Rico, and all five manufacturers of the injectable form of the drug have reported shortages of the product, according to a list maintained by the American Society of Health-System Pharmacists.

In a statement, Mylan did not address the methotrexate shortage but said it was “working closely with F.D.A. to help address drug shortage concerns.” Its plant has sources of water, electricity and communication, and the company said it is “working on ways to make them sustainable for manufacturing purposes.” Like other companies, it said it was focusing on helping employees and other residents of the island with basic needs, including chartering a cargo plane carrying essential goods to deliver to them.

Several pharmaceutical and medical device companies said that their factories were coming back on line with the assistance of generators and that they did not anticipate supply shortfalls. But others said the situation was precarious. Drug companies depend on consistent refrigeration to avoid shortages. And device makers are wondering when the power grid would be back up, which some officials have predicted could take months.