The Food and Drug Administration has decided to delay action on a closely watched Alzheimer’s drug, donanemab, which the agency was widely expected to approve this month. The F.D.A. will instead require donanemab to undergo the scrutiny of a panel of independent experts, the drug’s maker, Eli Lilly and Company, said Friday.

“The F.D.A. has informed Lilly it wants to further understand topics related to evaluating the safety and efficacy of donanemab, including the safety results in donanemab-treated patients and the efficacy implications of the unique trial design,” the company said in a statement.

The decision is likely to surprise many Alzheimer’s experts, doctors and patients who had expected the medication would soon be on the market. The F.D.A.’s move was startling to the company, which had been planning for the agency to greenlight the drug during the first quarter of this year.

“We were not expecting this,” Anne White, an executive vice president of Lilly and president of its neuroscience division, said in an interview. She said that while the F.D.A. often calls on such independent advisory committees when it has questions about drugs, it was unusual to do so “at the end of the review cycle and beyond the action date that the F.D.A. had given us.”

The F.D.A. did not say anything publicly about the move, which will delay any decision about whether to approve donanemab until at least later this year. Lilly officials said they expected it would be a few months before the advisory committee holds a hearing.

“The F.D.A. did commit to us to move quickly, so we would hope that they would then take action shortly after the advisory committee,” Mrs. White said.