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

This Phase 2 study had a randomized, multi-center, double-blind, placebo-controlled, crossover, multiple-administration design. The objectives of the study were the following: * To assess the multiple-dose analgesic efficacy and safety of an oral prolonged-release (PR) tablet formulation of GRT9906 at daily doses between 80 and 240 milligrams (mg) in comparison to placebo in participants with moderate to severe pain due to primary fibromyalgia syndrome (FMS). * To compare the tolerability of multiple-dose GRT9906 PR to placebo in participants with primary FMS. * To generate data that could be used, in combination with data from other studies, to explore the population pharmacokinetic analysis and pharmacokinetic/pharmacodynamic (PK/PD) properties of GRT9906 PR. The study consisted of 5 phases: 1. Enrollment including tapering, if necessary, and washout (at least 1 week) of previous medication. 2. First treatment period with 1-week titration and 5-weeks dosing on participant's last well-tolerated titration dosage. 3. Interim washout period of at least 1 week. 4. Second treatment period with 1-week titration and 5-weeks dosing on participant's last well-tolerated titration dosage. 5. Final washout period of at least 1 week, terminated by a Follow-up Visit.