Lack of recruitment.

Fibromyalgia is a cause of chronic pain, classified by the Internal Classification of Diseases (ICD) as a primary chronic pain with specific diagnostic criteria established by the American College of Rheumatology (ACR). No treatment to its complete cure is available at this time, all treatments having as purpose pain relief and an improvement of quality of life by combining pharmacologic and nonpharmacologic treatments. One of the mechanisms proposed in fibromyalgia is the central sensitisation phenomenon, by which the central nervous system becomes "hypersensitive" to nociceptive or non-nociceptive stimuli. The receptor involved in this phenomenon is the N-methyl-D-aspartate receptor to which ketamine binds. Ketamine has therefore been proposed as a co-treatment in chronic pain with central sensitization phenomena, such as fibromyalgia.

Ketamine is a drug used for anesthesia since the 1970s, acting as an antagonist to the N-methyl-D-aspartate receptor located in the central nervous system. It has two stereoisomers : R +ketamine and S - ketamine, the latter being about two to four times more potent. S-ketamine also has the advantages of producing less psychotropic effects, less tiredness and a lower temporary cognitive impairment than the racemic mixture at equianalgesic doses. Being first used as an anesthetic, in the last twenty years, ketamine's indications expanded to other domains, such as treatment of depression or off label use for chronic pain relief. Chronic pain is a multidimensional syndrome that touches 17-44% of the population of western countries. Fibromyalgia is a cause of chronic pain, classified by the Internal Classification of Diseases (ICD) as a primary chronic pain with specific diagnostic criteria established by the American College of Rheumatology (ACR). No treatment to its complete cure is available at this time, all treatments having as purpose pain relief and an improvement of quality of life by combining pharmacologic and nonpharmacologic treatments (as given by the European League Against Rheumatism (EULAR) guidelines). One of the mechanisms proposed in fibromyalgia is the central sensitisation phenomenon, by which the central nervous system becomes "hypersensitive" to nociceptive or non-nociceptive stimuli. The receptor involved in this phenomenon is the N-methyl-D-aspartate receptor to which ketamine binds. Ketamine has therefore been proposed as a co-treatment in chronic pain with central sensitization phenomena, such as fibromyalgia.

Inclusion Criteria:

• patients suffering from chronic pain (as defined by International Association for the Study of Pain) being diagnosed as fibromyalgia according to the American College of Rheumatology 2016 criteria
• patients qualified for the treatment by S-ketamine as established by our latest clinical practice
• patients aged 18-65 years old

The research pharmacist prepares the syringe with a placebo or active drug (group 1 with 0.2mg/kg or group 2 with 0.4 mg/kg of S-Ketamine) and anonymizes it before giving it to care provider.