In recent years, many anthocyanin-containing dietary supplements of various dosages and formulations have been sold through advertising their large number of beneficial effects. On the other hand, there is an increased risk of distributing deteriorated supplements to consumers due to lax regulations, because in Japan these supplements are classified as food. Spectrophotometric methods are commonly used to control the quality of supplements. However, these methods have limitations with regard to assessing deterioration. In this study, we evaluated a new index for detection of deteriorated products. The stability of 3 formulations and the quality of 20 supplements were investigated by ultra-high performance liquid chromatography, which is superior to spectrophotometry for identifying and quantifying individual anthocyanins. The stability was not only affected by storage temperature but also by formulation. We defined “Degradation Index” (DI) as an indicator of the deterioration of supplements. Of 20 supplements investigated, the DI of 5 supplements was more than 3-fold the value of Bilberon-25, and the worst one was 12.7-fold. These results suggest that DI could be a useful quality control index for detecting deteriorated supplements.