INTRODUCTION AND OBJECTIVES: The incidence of depression in women with breast cancer has been considerably high in recent years. The association of antidepressant drugs with hormone therapy drugs was considered effective and safe for both diseases. However, some new data has shown that concomitant use of SSRIs has increased the time on treatment for breast cancer and the incidence of death in these women. Fluoxetine is classically known to inhibit the serotonin (5-HT) reuptake, a monoamine with effects on the brain, intestine and breast cells, increasing its cellular activity. Our hypotheses for these effects observed in the clinic is that 5-HT signaling would be changed, interfering in the cancer progression. Thus, the aim of this study is to evaluate the effect of 5-HT and fluoxetine on breast cancer progression using in vivo and in vitro models, correlating them to the tumor phenotype. MATERIAL AND METHODS: For this, the MCF7 cell line, human breast cancer cell line was used for in vitro experiments using 10 µM 5-HT. The in vivo model was developed by inducing a xenograft tumor with the 4T1 cell line, murine breast cancer cell line, in 2-3 month old female BALB/c mice, which were treated for 21 days with 40 mg / kg fluoxetine. RESULTS AND CONCLUSIONS: Our results showed that the 5-HT-treated MCF7 cell line had a reduction in the breast cell hormone receptors for estrogen (ER), progesterone (PGR) and human epidermal fator (HER-2), showing a change in Luminal A type for a triple negative. Then, the main markers of the epithelium-mesenchymal transition were evaluated and it was observed an increase in the expression of the MMP9 enzyme, and the transcription factors SNAIL and TWIST, concomitantly with the reduction of the E-cadherin binding protein. The in vivo model showed that treatment with fluoxetine caused an increase in tumor size and an increasing lactate production by these tumors. In addition, the hormone receptor expression of these cells was investigated and no difference in ER, PGR and HER-2 mRNA expression was identified. At serum, it was identified a reduction in 5-HT level, due to fluoxetine treatment, which wasn’t observed in tumors. When investigating tumor metabolism, it was observed in fluoxetine treated tumors a high level of p-ACLY (Ser455) and p-ACC (Ser79). This increase was associated with higher levels of p-RICTOR (Thr1135) and p-mTOR (Ser2448), and p-AMPK (Thr172), respectively, which could be activated by different upstreams markers. Taken together, these results suggests that 5-HT contributes to increased breast cancer progression, and 5-HT induced by fluoxetine could alters signaling pathways, enhancing the cancer development.