Powerful drugs recently authorized by the U.S. that may prevent those at the earliest stages of COVID-19 from suffering severe disease present thorny new challenges, including who will get them and where they’ll be administered.

Antibody treatments, like one from Regeneron Pharmaceuticals Inc. that was used to treat President Donald Trump, are often administered to patients at their peak contagiousness. Regeneron’s drug, along with a therapy from Eli Lilly & Co., were authorized by the U.S. Food and Drug Administration for use within 10 days following patients’ first symptoms, and doctors will be racing against time to give them.

Though the U.S. has spent hundreds of millions of dollars to snap up the antibody treatments, they will be rationed due to limited supplies. Officials are working to establish sites to infuse the medications to patients with mild to moderate disease who had until recently been advised to stay home during an unprecedented surge in cases.