The first drug shown to slow Alzheimer’s disease hit the U.S. market over a year ago, but sales have lagged, major hospital systems have taken months to start using it and some insurers have rejected coverage.

Doctors also expect some patients will hesitate to take Leqembi due to its limited impact and potential side effects. They say it will take years to learn how best to deploy the drug and that work must be done to improve diagnosis.

“This is the start of a very exciting journey,” said Dr. Ambar Kulshreshtha, a suburban Atlanta family physician focused on dementia patients.

More than 6 million Americans, and millions more worldwide, have Alzheimer’s disease, the most common cause of dementia. There’s no cure, but Leqembi clears a sticky brain protein called amyloid that’s a key indicator of Alzheimer’s.

Studies showed that it can delay the progression of the disease by a few months when given to people with mild symptoms. Some experts say the delay may be too subtle for patients to notice.