Ectopic pregnancy (EP), which complicates 2% of all pregnancies, can be treated with either methotrexate (MTX) or surgery. While most cases can be managed with a single dose of MTX, the success rate has declined in recent years. One theoretical explanation for this decline might be related to smoking, as studies of patients with rheumatoid arthritis who smoke have demonstrated an inadequate response to MTX treatment. This study aims to identify risk factors for single-dose MTX treatment failure, with a focus on smoking status. We conducted a retrospective cohort study of patients diagnosed with EP and treated with single-dose MTX at a single institution between January 2001 and May 2021. The primary outcome was the incidence of failed MTX treatment, defined as the need for additional surgical intervention. Logistic regression was used to adjust the results and calculate the odds ratio (OR). The final analysis included 299 patients. Overall, 209 (69.9%) patients were treated successfully with a single dose of MTX, while 79 patients (26.4%) required further surgical intervention. A day 1 β-hCG level >
1500 mIU/mL and smoking were independently associated with MTX treatment failure, with an OR of 2.69 (95% confidence interval [CI] 1.41-5.13) and 2.58 (95% CI 1.16-5.75), respectively. Smoking and an initial β-hCG level above 1500 mIU/mL might reduce the success rate of single-dose MTX treatment for EP. Future studies should prospectively investigate alternative treatment protocols for patients with risk factors for medical treatment failure.