Auditing large, rapidly evolving terminological systems is still a challenge. In the case of RxNorm, a standardized nomenclature for clinical drugs, we argue that quality assurance processes can benefit from the recently released application programming interface (API) provided by RxNav. We demonstrate the usefulness of the API by performing a systematic comparison of alternative paths in the RxNorm graph, over several thousands of drug entities. This study revealed potential errors in RxNorm, currently under review. The results also prompted us to modify the implementation of RxNav to navigate the RxNorm graph more accurately. The RxNav web services API used in this experiment is robust and fast.