Background <p>Estrogen receptor alpha (ERα) plays a crucial role in the proliferation and survival of ER-positive breast cancer cells, and tamoxifen (TAM) remains the mainstay of endocrine therapy. However, acquired resistance to TAM remains a major clinical challenge. Despite advances in molecular profiling and targeted therapies, the mechanisms underlying TAM resistance remain incompletely understood. Emerging evidence suggest that dysregulation of intracellular signaling pathways, including the cAMP/CREB axis, may contribute to endocrine therapy failure.</p> Methods <p>We investigated the effects of cannabidiol (CBD) on TAM sensitivity and elucidated its underlying mechanisms in ER-positive breast cancer models. Human breast cancer cell lines (MCF7 and T47D) were treated with CBD, TAM or their combination. Apoptosis, proliferation, and protein expression were evaluated by flow cytometry, western blotting, and immunofluorescence. In vivo efficacy was examined in xenograft models.</p> Results <p>CBD significantly enhanced TAM-induced cell death and apoptosis in ER-positive breast cancer cell. Mechanistically, CBD suppressed the cAMP/CREB signaling pathway, leading to downregulation of ERα and its target genes (<i>TFF1</i>,<i> GREB1</i>,<i> CCND1</i>). Co-immunoprecipitation revealed that CBD inhibited the interaction between phosphorylated CREB and CBP, resulting in transcriptional suppression of ERα. In vivo, combined treatment with CBD and TAM synergistically inhibited tumor growth and reduced ERα and p-CREB expression levels in tumor tissues.</p> Conclusions <p>Our findings demonstrate that CBD restores TAM sensitivity through CREB-mediated downregulation of ERα signaling in ER-positive breast cancer. This study suggests the potential application of CBD as a novel adjuvant agent to overcome TAM resistance and improve therapeutic outcomes in patients with ER-positive breast cancer.</p> Graphical abstract <p></p>