Last week, two high-profile San Francisco health start-ups began marketing at-home coronavirus kits that let people collect their own saliva samples or oral throat swabs and then send the specimens to commercial labs to be tested for the virus.

The start-ups, Carbon Health and Nurx, each said they were preparing to offer thousands of the kits in the coming weeks. By Friday afternoon, the Nurx site said its kits had “reached capacity for today” and promised more would be available for sale this week.

But on Friday evening, the Food and Drug Administration issued an alert warning consumers that it had “not authorized any test” for the coronavirus that people could buy and administer at home. Carbon Health and Nurx subsequently suspended sales of their kits.

As the coronavirus pandemic intensified across the country, the two companies and other start-ups rushed to market collect-your-own specimen kits without rigorous published studies proving the effectiveness of at-home swabbing for coronavirus testing.