The Food and Drug Administration is worried that there could be shortages of some critical drugs normally produced in Puerto Rico if manufacturing plants there are closed for an extended period of time.

Dr. Scott Gottlieb, the commissioner of the FDA since May, arrived on Puerto Rico Friday to survey the situation and talk with pharmaceutical executives on the island. He said his agency has a list of about 40 drugs that it's concerned could be in short supply, 13 of which are made only in Puerto Rico.

"These are critical medicines. These are not drugs for which there are therapeutic substitutes," he told reporters on the flight to Puerto Rico Friday.

Related: Hurricane Maria could be a $95 billion blow to Puerto Rico

The FDA is also working to make sure that island residents can get the drugs they need.

There are about 50 pharmaceutical plants on the island and about 30 that make medical devices. The industry has about 90,000 jobs there, and accounts for about 75% of the goods shipped off the island. Most major pharmaceutical companies have some kind of manufacturing presence on the island, including Bristol-Myers Squibb (BMY), Eli Lilly (LLY) and Amgen (AMGN).

Gottleib said it's important that the plants that make drugs and medical devices resume normal operations soon, not just to help the patients who need them, but for Puerto Rico's economic health.

"That manufacturing base is critical to long-term manufacturing success of the island, and also getting people back to work," Gottlieb said.

Related: Who will help Puerto Rico?

Bristol-Myers, Lilly and Amgen all told CNNMoney that they have adequate product inventory and there are not currently any shortages for their customers. They all said their major concern is the welfare of their Puerto Rican employees.

The good news is that the major factories suffered little damage, and most operating to some degree on generator power. The FDA's concern is making sure that those plants continue to get the fuel they need for the generators, the supplies they need to produce the drugs, and that the products they make can get off the island.

"We think we're getting ahead of it," he said about the potential for shortages. "The situation has been improving a lot since last week."

Another challenge to keeping the factories running: Many employees haven't been able to return to work since Hurricane Maria hit nine days ago.

"People are still taking care of their families," Gottleib said.