Key Points Question How often do randomized clinical trials directly compare new single-enantiomer drugs to their existing racemic precursors, and how often are efficacy or safety differences observed? Findings In this systematic review of 15 single-enantiomer racemic drug pairs, 185 direct-comparison randomized clinical trials (median, 2 trials; interquartile range, 1-8 trials) were identified, 124 (67.0%) of which studied 1 drug pair. For 9 single-enantiomer drugs, no randomized clinical trials were identified providing evidence of improved efficacy, based on primary end point results, or safety as compared with their racemic precursors. Meaning Results of this systematic review suggest that most newly marketed single-enantiomer drugs are infrequently directly compared with their existing racemic precursors, and when compared, they are uncommonly found to provide improved efficacy or safety, despite their greater costs.

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