Efficacy and Safety of GRT9906 Tablets Compared to Placebo in Patients With Fibromyalgia

Inclusion Criteria: * Male or female ambulatory participants of any ethnic group, aged 18 to 75 years inclusive at enrollment. * Primary fibromyalgia syndrome (FMS) diagnosed according to the American College of Rheumatology (ACR) 1990 criteria, persistent for at least 6 months. * Average Pain Intensity of FMS pain over the last 3 days before randomization visit must be at least 4 points, using 11-point numerical rating scale (NRS). * Negative urine test for drugs of abuse at the Day 1 visit of each treatment period. * Women of childbearing potential must use an acceptable method of contraception (i.e., double barrier, hormonal or intra-uterine device method) during the study period and have a negative urine pregnancy test at the Enrollment Visit and at the Day 1 visit of each treatment period. * Compliance with use and completion of assessments by means of electronic diaries; 80 percent of entries must be available for the week before randomization. * Written informed consent for study participation given. Exclusion Criteria: * Participation in another study of IMPs or devices parallel to, or less than 1 month prior to enrollment, or previous participation in this study. * Known to or suspected of not being able to comply with the study protocol and the use of the IMPs. * Not able to communicate meaningfully with the Investigator and staff. * Evidence or history of alcohol, medication or drug dependency during the past 12 months. History of opiate dependency at any point in life. * Evidence or history of neurotic personality, psychiatric illness including anxiety disorder, severe senile dementia, Alzheimer's disease, history of seizures or pre-existing conditions that lower seizure threshold (e.g., head trauma), or suicide risk. * Current depression needing treatment with antidepressants. * Currently or previously diagnosed with malignancies except basal cell carcinoma; poor medical status (e.g., New York Heart Association \[NYHA\] class equal to or above 3; Child classification for hepatic impairment above A \[Pugh et al. 1973\]; decompensated chronic obstructive pulmonary disease) or, at the discretion of the Investigator, clinical signs that raise concerns about participant's suitability for the study. * Creatinine higher than 1.5-times of upper limit of normal (ULN) range. * Alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) higher than twice the ULN range. * Any chronic disease (e.g., hepatic, renal and/or gastrointestinal) that m…