Nine months into the U.S. launch of the first drug proven to slow the advance of Alzheimer's, Eisai and Biogen's Leqembi is facing an unexpected hurdle to widespread use: an entrenched belief among some doctors that treating the memory-robbing disease is futile.

Alzheimer's experts had anticipated bottlenecks due to Leqembi's requirements, which include additional diagnostic tests, twice-monthly infusions and regular brain scans to guard against potentially lethal side effects.

And those issues have played a role in slow adoption since the drug was approved by the U.S. Food and Drug Administration, according to interviews with 20 neurologists and geriatricians from rural, urban, academic and community practices in 19 states.