<p>Depressive symptoms are recognized contributors to breast cancer progression; while previous studies have investigated the role of antidepressants in breast cancer, their therapeutic potential remains insufficiently and systematically explored. This study systematically evaluated the anti-tumor efficacy of six clinically used antidepressants (escitalopram, amitriptyline, fluoxetine, paroxetine, desvenlafaxine, and duloxetine) in 4T1 (murine triple negative) and MCF-7 (human ER⁺/HER2⁻) breast cancer cells. The results indicated that duloxetine exhibited the most potent cytotoxicity, with IC₅₀ values of 6.94 μM in 4T1 and 16.16 μM in MCF-7 cells. Although less potent than docetaxel, duloxetine still exhibited significant anti-proliferative, anti-migratory, and pro-apoptotic effects. In vivo, duloxetine (10–30 mg/kg) significantly suppressed tumor growth in 4T1-bearing mice, accompanied by decreased Ki-67 expression and increased E-cadherin. Moreover, duloxetine significantly inhibited the phosphorylation of AKT and mTOR, thereby promoting autophagy activation, as evidenced by increased LC3 expression and decreased P62 levels. Additionally, duloxetine promotes apoptosis by upregulating Bax and downregulating Bcl-2 expression both in vitro and in vivo. These findings reveal that duloxetine exerts anti-breast cancer effects through inhibiting AKT/mTOR signaling and promoting Bax/Bcl-2-mediated apoptosis, highlighting its potential for therapeutic repurposing as an adjunct treatment—particularly, in breast cancer patients with coexisting depression.</p>