The article titled Sildenafil citrate (Viagra) impairs fertilization and early embryo development in mice by Glenn et al. (1) was of great interest. However, the presented data should be clarified from a teratological point of view, as it is insufficient for any further toxicological evaluation, in particular the human risk assessment. Without recording the initial number of females selected to each examined group, it is impossible to calculate the mating and fertility indexes that are crucial for stage A of developmental toxicological studies (2). In addition, the postimplantation loss on day 4 should be calculated.