Thus, the main objectives of this study are: (i) to develop an effervescent OC dosage form with effective taste masking and enhanced dissolution; (ii) to assess the efficacy of the formulated effervescent powder against the growth of the estrogen receptor positive/human epidermal receptor-2 positive (ER+/HER2+) BC in vitro and in vivo in nude mouse xenograft models; (iii) to determine the ER+/HER2+ BC locoregional recurrence inhibitory efficacy of the formulated effervescent powder in an orthotopic xenograft mouse model after primary tumor surgical excision. The gene discussed is ESR1; the disease is breast cancer.