Since last summer, Medicare has been evaluating whether to pay for a newly approved, exorbitantly priced drug to treat Alzheimer’s disease. Its decision was always going to be fraught: The data on the drug’s potential benefits are ambiguous at best, and its risks are considerable. About 40% of patients who have taken it have suffered swelling or bleeding in the brain. Others have experienced disabling nausea, dizziness, headaches and confusion. Biogen, the manufacturer, is investigating a patient death.