Current regulation for generic (biosimilar) approval is based on the assessment of average bioequivalence (biosimilarity) based on interval hypothesis testing or Shuirmann’s two one-sided test procedure. As indicated by the United States Food and Drug Administration (FDA), an approved generic (biosimilar) drug can be used as a substitute for the innovative drug. FDA does not indicate that two generic copies of the same innovative drug can be used interchangeably even though they are bioequivalent (biosimilar) to the same innovative drug. In practice, bioequivalence (biosimilarity) between generic copies of an innovative drug is not required. However, as more generic (biosimilar) drug products become available, it is a concern whether the approved generic drug products have the same quality and therapeutic effect as the innovative drug product and whether they can be used safely and interchangeably. In this presentation, several criteria including a newly proposed criterion for assessing drug interchangeability are studied. In addition, comments on possible study designs and power calculation for sample size under a specific design are also discussed.