Major changes to prescribing instructions are rare, especially so soon after approval; change could curb the drug's use
FDA approved Aduhelm earlier this month, despite advisory committee's conclusion that there was not enough evidence to support its effectiveness
Donanemab has already received a 'breakthrough therapy' designation from the FDA
Several experts have said the available evidence about Aduhelm raises significant doubts about its effectiveness
Agency says it has granted approval on the condition that drug maker Biogen conduct a new clinical trial