What is a key limitation of the Prevalent User Design?

• A. Confounding by indication
• B. Survivor bias
• C. Loss to follow-up
• D. Information bias

Answer: (B)

The main idea here is that the prevalent user design looks at people who are already on a therapy at the start of the study. Because those individuals have already tolerated and continued the treatment, this design misses the early period after initiation when adverse effects or discontinuation often occur. That means the sample is enriched for “survivors” who have not only tolerated the drug but may appear to fare better over time, producing a biased impression of safety and effectiveness. This survivor bias inflates the perceived benefits and understates early risks, since any events happening during the initial treatment period aren’t captured.

Other biases like confounding by indication, loss to follow-up, or information bias can arise in various observational setups, but they are not the defining limitation of the prevalent user approach. The key issue specific to this design is the selection of long-term users who have already survived the initial, potentially problematic period after starting treatment.