What were Aduhelm's safety risks?
About 41% of patients on the high dose experienced amyloid-related imaging abnormalities including brain swelling in 35% — all requiring repeated MRI monitoring — while uncertain benefit left many reviewers concluding the risk profile was hard to justify.
The main risk was amyloid-related imaging abnormalities, or ARIA, which includes brain swelling and small bleeds visible on MRI. A secondary analysis of the phase 3 trials published in JAMA Neurology found that about 41% of patients on the high dose experienced ARIA, with brain swelling (ARIA-edema) affecting 35%. About a quarter of those with ARIA-edema had symptoms, most commonly headache, confusion, dizziness and nausea, and microhaemorrhages affected about 19%. Because ARIA requires regular MRI monitoring, the drug's real cost included scans and specialist visits on top of the infusion. Medicare cited the potential for harm when restricting coverage. Supporters noted most ARIA was asymptomatic and resolved, but the risk profile, combined with uncertain benefit, weighed heavily in the backlash.