What strategy helps reduce bias from prior drug use when studying new users?

• A. Include All Users Regardless Of History
• B. Exclude Long-Term Users When Studying New Users
• C. Exclude All Ever-Users
• D. Only Study Current Users

Answer: (B)

Focus on starting exposure. When studying people who are just beginning a drug, including long-term or prevalent users introduces bias from prior exposure. Those long-term users differ in important ways—health status, reasons for continuing treatment, tolerance to the drug, adherence, and competing risks—that can distort the association you’re trying to measure about new use. This is called prevalent-user bias: you’re mixing people who already tolerated or benefited from the drug with true initiators, which can underestimate risks or misrepresent early effects.

Excluding long-term users and starting the observation at initiation keeps the comparison more balanced. You follow new initiators from the moment they start and compare them to those not yet exposed, so baseline characteristics and risk factors related to prior use are not confounding the results. This approach helps isolate the effects that occur with the act of starting the drug, rather than effects shaped by a long history of prior use.