Blinding, also called masking, is a methodological technique designed to minimize bias in clinical trial results by preventing knowledge of treatment assignments from influencing behavior or assessments. When successful, blinding ensures that expectations about treatment effects do not consciously or unconsciously affect how participants report symptoms, how investigators evaluate outcomes, or how data analysts interpret results. This protection against bias is essential for generating credible evidence about treatment effects.

Single-blind trials typically keep participants unaware of their treatment assignment while investigators know which treatment each participant receives. Double-blind trials extend this protection to include the investigator and often the study personnel conducting assessments. Triple-blind designs additionally mask the treatment assignment from data analysts, data monitoring committees, or other parties. The appropriate level of blinding depends on the nature of the endpoints being assessed and the feasibility of maintaining the blind given the characteristics of the treatments being compared.

Maintaining the blind throughout a trial requires careful attention to many details. Investigational products and placebo must be identical in appearance, taste, smell, and packaging. Procedures must prevent inadvertent unblinding through laboratory values, side effects, or other clues. Emergency unblinding procedures must be available when knowledge of treatment assignment is medically necessary, but these procedures must be carefully controlled to prevent inappropriate unblinding. The integrity of blinding should be assessed, as premature or frequent unblinding can compromise the validity of trial results.