There’s little evidence that the drug, Aduhelm, slows the progression of dementia, but the Food and Drug Administration approved it this month. Analysts expect that Medicare and its enrollees, who pay a share of their prescription drug costs, will spend $5.8 billion to $29 billion on the drug in a single year. “It’s unfathomable,” said Tricia Neuman, executive director of the Kaiser Family Foundation’s program on Medicare policy. “These are crazy numbers.” Plenty of other drugs cost more than Aduhelm, which is made by Biogen and will be priced at $56,000 annually. What makes it different is that there are millions of potential customers, and the drug is expected to be taken for years.