Magnesium, Stress and Fibromyalgia

Inclusion Criteria: * Patient over the age of 18, * Patient with fibromyalgia according to the criteria of the 2016 ACR, * Patient presenting a score on the DASS-42 \> 18 scale, * Patient free from any initiation of new treatment or diet at the time of inclusion, * Cooperation and understanding sufficient to comply with the requirements of the study, * Acceptance to give written consent, * Affiliation to the French Social Security system, * Registration or acceptance of registration in the National Register of Volunteers participating in research. Exclusion Criteria: * having a contraindication to the administration of magnesium: hypersensitivity to magnesium chloride, or to any of the excipients, * with a magnesium concentration of \> 1.05 mmol / l,- with severe renal impairment with a creatinine clearance \< 30 ml / min, * receiving a treatment or dietary supplement containing magnesium at the time of inclusion, * treated with antibiotics in the three months prior to inclusion,- who has reported gastroenteritis in the two months prior to inclusion, * physically unfit to place the palms or soles of the feet on the Sudoscan® electrodes, * having a medical and / or surgical history judged by the investigator or his representative to be incompatible with the test (in particular a disease known to cause small fiber neuropathies: diabetes, Sjögren's disease, vasculitis, sarcoidosis, chronic ethylism ....), * using anticoagulant therapy or allergy to local anesthetics,- of chil…