In breast cancer patients undergoing a mastectomy, the aim of this study is to evaluate if the magnesium administered for 6 weeks starting two weeks before the surgery induces a decrease of pain intensity one month post-mastectomy compared to the placebo group.

This is a randomized, placebo-controlled double-blind clinical trial conducted in the Oncology Hospital, Clermont-Ferrand, France, in 100 patients suffering from breast cancer and undergoing a total mastectomy.

This clinical trial compares magnesium and placebo for the prevention of neuropathic pain induced by mastectomy assessed with a (0-10) numerical pain rating scale. Cognition, anxiety, depression, sleep and quality of life are also assessed.

Inclusion Criteria:

• - Age ≥ 18 years,
• Patient with breast cancer who has a scheduled total mastectomy with or without treatment two weeks after inclusion with or without preoperative chemotherapy,
• Patient free from any new treatment or diet at the time of the inclusion,
• Cooperation and understanding sufficient to comply with the requirements of the study,
• Patients affiliated to the French Social Security,
• Patients with free and informed consent has been obtained

Exclusion Criteria:

• Hypersensitivity to the active substance or to any of the excipients
• Patient with magnesemia >1,05 mmol/l
• Patient with severe renal insufficiency with creatinine clearance <30 ml min,
• Patient with an addiction to alcohol, as determined by the investigator,
• Diabetes (type I and II),
• Medical and surgical history incompatible with the study,
• Patient receiving treatment with Quinidine, L-Dopa,
• Childbearing age, no use of effective contraceptive method, pregnancy or lactation
• Patient exclusion period, or the total allowable compensation exceeded
• Patients undergoing a measure of legal protection (guardianship, supervision ...)