Smoking worsens breast cancer prognosis. It has been reported that tobacco components directly reach the mammary gland tissue, causing smoking-related DNA damage and biological effects. We hypothesized that smoking may have characteristics that affect the therapeutic effect and clinical course in patients with stage IV hormone receptor-positive breast cancer (HRBC) who received endocrine therapy as the first-line treatment.

Fifty-six patients diagnosed with stage IV HRBC were treated with endocrine therapy as the first-line treatment. Before treatment, the period and amount of smoking were confirmed through patient interviews, and each pack-year was recorded.

Disease progression with new metastases was more frequent in smokers than non-smokers during endocrine therapy as first-line treatment (p=0.034). Furthermore, as the pack-year increased, new metastases tended to appear [pack-year ≤15; hazard ratio (HR)=1.929, p=0.507; pack-year 15-30, HR=3.857, p=0.223; pack-year >30, HR=7.714, p=0.028].

In stage IV HRBC, smoking increases the metastatic potential of breast cancer, suggesting that changes in its biology may lead to poor prognosis.