Objective: The aim of this study was to assess the synergistic effect of non-adherence to the Mediterranean Diet (MD) and lifestyle habits on the occurrence of breast cancer (BC). Patients and methods: A case-control study was carried out from September 2018 to February 2019 at the Teaching Hospital "Umberto I" in Rome. A Food Frequency Questionnaire was used for assessing the level of adherence to MD, the IPAQ Questionnaire to measure physical activity, and AUDIT-C to estimate alcohol consumption. The possible interaction between risk factors was tested using the synergism index. Results: A total of 94 cases and 88 controls were enrolled (median age 55.8 for cases and 57.9 for controls). The MD Score over 6 was associated with low odds of having breast cancer (OR = 0.29; 95% CI: 0.12-0.69). There is a clear indication for the additivity and synergism between non-adherence to MD and many risk factors on the occurrence of BC: current smoker (S = 2.02; 95% CI 0.62-8.07), physical inactivity (S = 2.14; 95% CI 0.71 2-8.28) and alcohol consumption (S = 3.02; 95% CI 0.91-12.95). Conclusions: Primary prevention of BC can benefit from intervention targeting nutritional and lifestyle factors that act synergistically
