In Mexico, breast cancer (BC) is the principal cause of death in women over 30 years old, with an annual mortality rate of 14.61 deaths per a 100,000 population. Chemotherapy, in combination with trastuzumab (TTZ), improves the survival of cancer patients; however, cardiotoxicity (CT) is the principal consequence. CT prevalence occurs between 10% and 30% of patients; however, there are no data about the prevalence of CT in the Mexican population. This study aims to establish the prevalence of CT in patients treated with anti-HER-2 therapy among BC women in southeast Mexico. A retrospective cross-sectional study was carried out from January 2015 to July 2019. The records of 46 patients diagnosed with HER-2-positive BC who attended the Mexican Social Security Institute in the Ambulatory Care Medicine Unit were analyzed. The diagnostic criterion for CT was a decrease in LVEF > 10% from baseline or a final LVEF < 53%. CT prevalence was observed in 19 (41.3%) of women with cancer, with an average decrease in LVEF of 13%. In the population, we found an association between weight, surface area, and the loading dose of TTZ with CT. Nutritional follow-up and the administration of cardioprotective drugs are necessary to recover LVEF and avoid cardiovascular failure in women with BC and survivors.