On his website, Consultox.com, forensic toxicology expert witness Dr. Roger A Parent writes on generic drugs:
Acceptable limits of impurities may be specified in the United States Pharmacopeia (USP) and may be considered in the qualification of an impurity. If the impurity does not appear in the USP monographs, you may qualify it by comparison with the observed amount of that impurity as found in the reference listed drug product (RLD). As an alternative, the impurity may be qualified by justification in the scientific literature, metabolite data, or with toxicity studies. An impurity is considered qualified when it meets one or more of the following conditions.
* The observed level and proposed acceptance criterion for the impurity do not exceed the level observed in the referenced listed drug product.
* The impurity is a significant metabolite of the drug substance.
* The observed level and proposed acceptance criterion for the impurity are adequately justified by scientific literature.
* The observed level and proposed acceptance criterion for the impurity do not exceed the level that has been adequately evaluated in toxicology studies.
Quantitative Structure Activity Relationships(QSAR) programs may be considered for predicting toxicity, but the results are not considered conclusive for the qualification process.