A federal panel of medical experts on Friday sharply rejected arguments in support of a closely watched new Alzheimer’s drug, saying the evidence wasn’t persuasive enough for the drug to be approved as the first new Alzheimer’s therapy in nearly two decades.

The nonbinding vote by an advisory panel for the Food and Drug Administration does not mean the agency won’t approve the drug, aducanumab, made by Biogen. But it signals that many experts in the field are not convinced of its effectiveness, another major setback in the long journey to find a treatment for Alzheimer’s that works.

In a seven-hour virtual meeting Friday, the panel showed pointed skepticism, which contrasted markedly with a presentation by Dr. Billy Dunn, director of the FDA’s office of neuroscience, who said that “the evidence supporting its approval appears strong.”

Overwhelmingly, the panel members disagreed.

Ten of 11 panel members voted that it was not “reasonable” to consider the research presented as “primary evidence of effectiveness of aducanumab for the treatment of Alzheimer’s disease.” The 11th member said he was uncertain.

It was a stunning turn of events, coming two days after the FDA had posted documents that suggested most of the agency’s reviewers considered the evidence convincing, a development that sent Biogen’s stock price soaring by more than 40%. Most of Friday’s votes occurred after the financial markets had closed.

Nearly 6 million people in the United States and roughly 30 million globally have Alzheimer’s disease, a number that is expected to more than double by 2050. Aducanumab — which is given as a monthly intravenous infusion and would cost about $50,000 per year — would be the first medication to address cognitive decline by attacking the core biology of Alzheimer’s disease. It has been considered a potential medication for the roughly 2 million Americans estimated to have mild Alzheimer’s-related cognitive decline.

The drug’s path through clinical trials has been rocky, with only one of two phase 3 trials showing positive results — and those results emerged only from an analysis of additional data after the trials were stopped in March 2019 by an independent data monitoring committee because the drug didn’t appear to be working.

The FDA usually requires two convincing studies for a drug to be approved, although the agency has made exceptions, especially for severe or deadly diseases for which little or no treatment is available.