Author Response: Prescribed Drug Use and Aneurysmal Subarachnoid Hemorrhage Incidence: A Drug-Wide Association Study

We thank Drs. Cameron and Siegler for their thoughtful letter on our article.1 We think that the first point on the increased risk of aneurysmal subarachnoid hemorrhage (aSAH) with prochlorperazine use, and the possible explanation with sentinel headaches, concerns protopathic bias, or reverse causality. This bias occurs when the early signs or symptoms of a still undiagnosed latent outcome lead to prescription of a medicine and then to a flawed association between the prescribed medicine and that outcome, when it is finally diagnosed.2 This usually happens with chronic outcomes that have a long latency period, such as cancers.3