Purpose: Evaluate whether morbid obesity influenced resolution, number of doses or ultimately surgical management of tubal ectopic pregnancy (TEP) when treated with single-dose regimen methotrexate (SDR-MTX) capped at 100 mg. Methods: Retrospective cohort study of patients with a diagnosis of TEP who underwent MTX treatment from 2000 to 2013. Patients were excluded if initial β–hCG <1000 mIU/mL, did not have β–hCG follow-up or were not treated with SDR-MTX. Per protocol, dose was administered at 50 mg/m2 with a capped maximum of 100 mg. Patients were divided based on their BMI (<40 and ≥40 kg/m2). Demographic variables, β–hCG before treatment, maximum diameter of ectopic size, embryonic heart tones, decrease of β–hCG, need for additional MTX doses and surgery despite treatment were recorded and compared among the groups. Results: 151 women were included in the study, 89.4% (135/151) non-morbidly obese and 10.6% (16/151) morbidly obese. No differences in age distribution, ethnicity, pre-treatment presence of embryonic heart tones, maximum diameter of ectopic size ≥35 mm and β–hCG ≥5000 mIU/ml were found. Following treatment, the proportion of patients with at least an 80% decrease in their β–hCG levels or need for surgery were similar, however, morbidly obese patients were significantly more likely [11/134 vs. 5/16, OR 5.1 (1.5–17.3, p = 0.015)] to require an additional MTX dose. Conclusion: Among patients with TEP, morbidly obese patients were five times more likely to require an additional dose compared to non-morbidly obese when SDR-MTX capped at 100 mg was used for medical management.