A preliminary phase at the beginning of a clinical trial, before randomization, during which all participants receive either placebo or the active treatment. Run-in periods serve several purposes: establishing baseline measurements, screening out participants who cannot tolerate the treatment or adhere to the protocol, and stabilizing patients before the experimental phase begins.

Run-in periods can significantly affect trial results. A placebo run-in that eliminates “placebo responders” before randomization can make the treatment look more effective than it would in an unselected population. An active run-in that eliminates patients who experience side effects inflates the apparent safety profile. When evaluating trial data, note whether a run-in period was used and what criteria were applied—this context matters for interpreting the results.