The aim was to perform a comparative evaluation of composition and in vitro release performance of multisource acyclovir 5% creams. Significance: The outcome was analysed in relation with the principles of the Topical drug Classification System (TCS). Methods: The in vitro drug release testing (IVRT) was based on selection of an inert artificial membrane and a medium providing sink conditions, and utilizing the vertical diffusion cells. US and European innovator products, with marked difference in excipients, were used as references for the assessment of the in vitro release similarity. The qualitative composition of the topical semisolid products was inventoried, with no quantitative details being available. A Principal Component Analysis was applied by either dichotomy ranking or grouping the individual excipients into categories according to their functional role. Results: The results confirmed the sensitivity and discriminative characteristics of IVRT with respect to the qualitative composition, as well as its relevance in the comparative assessment of multisource drug products beyond the current strict requirements of Q1 and Q2 similarity. Conclusions: This is in line with the principles of the TCS and with the central role assigned to IVRT.