WASHINGTON — When a controversial Alzheimer’s drug won U.S. approval, surprise over the decision quickly turned to shock at how long it might take to find out if it really works — nine years.

Drugmaker Biogen has until 2030 to complete a study confirming whether its new drug Aduhelm truly slows the brain-destroying disease. That’s under the terms of the Food and Drug Administration’s conditional approva l of the drug, a decision that has been praised by patients as overdue and condemned by the agency’s own outside experts.

But both camps agree: 2030 is far too long to wait for answers on the $56,000-a-year drug.

“We think nine years is unacceptable and our expectation is that it will happen in a much shorter time frame,” said Maria Carrillo of the Alzheimer’s Association, an advocacy group that pushed for approval but now wants the FDA to set a quicker deadline.

Other experts warn that the 2030 timeline could slip if patients balk at enrolling in a new study for a drug that’s already available. And the focus on Aduhelm — the first new Alzheimer’s drug in 18 years — could steer volunteers away from testing of other promising treatments.

“If someone can go to their physician and get the FDA-approved drug, why would they go into a trial where they risk getting a placebo?” said Donna Wilcock, an Alzheimer’s researcher at the University of Kentucky.

To establish a new drug’s safety and effectiveness, researchers compare results in people who get the treatment to a similar group of people who don’t. That generally means half of the volunteers are randomly assigned to get a fake treatment instead of the real thing.

Biogen already conducted two such large studies of its drug, which requires monthly IVs. The studies took about four years to run and followed participants for about 1 1/2 years. Both were stopped early when it seemed the drug wasn’t working, and the results were so mired by flaws and inconsistencies that the FDA deemed them too weak to support approval on the basis of slowing the disease.

Instead, the agency took another approach and gave the drug conditional approval based on a promising sign: its success in getting rid of a buildup of sticky plaque in the brain that is thought to play a role in Alzheimer’s disease.

Under its so-called accelerated approval program, the FDA is requiring Biogen to conduct a new study definitively answering whether Aduhelm’s effect on plaque truly slows mental decline in patients. Other Alzheimer’s drugs on the market only temporarily ease symptoms.

The FDA has not detailed how the 2030 target came about, or why such a distant deadline was granted for a drug that could be given to millions of patients in the coming years, adding billions to the nation’s health care bill.