Visceral leishmaniasis (VL) is the most severe form of leishmaniasis and is caused by Leishmania infantum in the Americas. Since the use of Milteforam™ was authorized to treat canine visceral leishmaniasis (CVL) in Brazil in 2017, there has also been fear of the emergence of parasites resistant to this drug and, through cross-resistance mechanisms, to meglumine antimoniate and amphotericin B. Additionally, the literature shows that acquisition of resistance is followed by increased parasite fitness, with higher rates of proliferation, infectivity and metacyclogenesis, which are determining factors for parasite virulence. In this context, this study aims to analyze the impact of treating a dog with Milteforan™ on the generation of parasites resistant to miltefosine, meglumine antimoniate, and amphotericin B. To this end, in vitro susceptibility tests were conducted against these drugs with T0 (parasites isolated from the dog before treatment with Milteforan™), T1 (after one course of treatment), and T2 (after two courses of treatment) isolates. The rates of cell proliferation, infectivity, and metacyclogenesis of the isolates were also evaluated. The results indicate a gradual increase in parasite resistance to miltefosine and amphotericin B with increasing the number of treatment courses. A trend increase in the metacyclogenesis rate of the parasites was also observed as drug resistance increased. Therefore, treatment of CVL with Milteforan™ induces resistance to miltefosine and amphotericin B as well as changes in parasite fitness, and may have an impact on animal and human public health.