What were Aduhelm's safety risks?
About 41% of patients on the high dose experienced amyloid-related imaging abnormalities, including brain swelling in 35% and microhaemorrhages in 19%, requiring repeated MRI monitoring throughout treatment.
The main safety concern was amyloid-related imaging abnormalities (ARIA), which include brain swelling and small bleeds visible on MRI. A secondary analysis of the two phase 3 trials, published in JAMA Neurology in November 2021, found that about 41% of patients on the high dose experienced ARIA, with brain swelling affecting 35%. Roughly a quarter of patients with swelling had symptoms, most commonly headache, confusion, dizziness and nausea, and microhaemorrhages affected about 19%.
Because ARIA must be monitored with repeated MRI scans, the true cost and burden of treatment extended well beyond the drug itself. When Medicare restricted coverage to clinical trials, it cited the potential for harm from significant unknowns. Supporters noted that most ARIA was asymptomatic and resolved, but critics argued the risks were hard to justify given the uncertain benefit.