Inclusion Criteria:

• Age ≥ 18 years
• Able to give written informed consent
• Able to complete subject-reported questionnaires per the investigator's judgment
• At screening, subjects must meet the 1990 American College of Rheumatology (ACR) criteria for FM, i.e. widespread pain present for at least 3 months and pain in at least 11 of 18 specific tender point sites. In addition, the 2010 ACR criteria must be met:
• Widespread pain index (WPI) ≥ 7 and symptom severity (SS) scale score ≥ 5, or WPI 3 to 6 and SS scale score ≥ 9
• Symptoms have been present at a similar level for at least 3 months
• The subject does not have a disorder that would otherwise explain the pain
• ADPS of ≥ 4 on the 11-point numeric rating scale (NRS) over the past 7 days prior to randomization (based on completion of at least 4 daily pain diaries during the 7-day baseline period prior to randomization)
• Subject must have documented evidence of a fundoscopic examination (with pupil dilation) within 12 months prior to screening or at screening.
• Women of child-bearing potential (WOCBP) must be using an adequate method of contraception to avoid pregnancy during the study and for 4 weeks after study completion.

Exclusion Criteria:

• Clinically significant unstable neurologic, psychiatric, ophthalmologic, hepatobiliary, respiratory, or hematologic illness or unstable cardiovascular disease (e.g. severe hypotension, uncontrolled cardiac arrhythmia, or myocardial infarction) or any other concurrent disease within 12 months prior to screening that in the opinion of the investigator would interfere with study participation or assessment of safety and tolerability
• Anticipation of initiation or significant change to normal daily exercise routines or need for ongoing use of concomitant medications or non-pharmacological pain management techniques that may confound assessments of efficacy and/or safety
• Unable to undergo pre-study washout of prohibited concomitant medications
• Subjects who are at risk of suicide as defined by their responses to the Columbia-Suicide Severity Rating Scale (C-SSRS) or in the opinion of the investigator. Note: Patients answering "yes" to any of the questions about active suicidal ideation/intent/behaviors occurring within the past 12 months must be excluded (C-SSRS Suicide Ideation section - Questions 3, 4, or 5; C-SSRS Suicidal Behavior section, any of the suicide behaviors questions). Such patients should be referred immediately to a mental health professional for appropriate evaluation.
• Current severe or uncontrolled major depressive disorder or anxiety disorders as assessed by the Mini-international Neuropsychiatric Interview (MINI) mild to moderate major depression or anxiety disorders are permitted provided that the investigator assesses the patient as clinically stable and appropriate for entry into the study.
• Any diagnosis of lifetime bipolar disorder or psychotic disorder
• Subjects with pain due to other conditions (e.g. diabetic peripheral neuropathic pain or post-herpetic neuralgia) that in the opinion of the investigator, would confound assessment or self-evaluation of the pain associated with FM.
• Subjects with pain due to any widespread inflammatory musculoskeletal disorder (e.g. rheumatoid arthritis, lupus) or widespread rheumatic disease other than FM.
• Abuse or dependence of prescription medications, street drugs, or alcohol within the last 1 year
• Any history of a malignancy other than basal cell carcinoma within the past 5 years
• A diagnosis of untreated sleep apnea or initiation of treatment for sleep apnea within the past 3 months
• Pregnancy or breast-feeding, or intent to become pregnant during the study period
• Subject is currently enrolled in or has not yet completed at least 30 days since ending another investigational device or drug study or is receiving other investigational agents.
• Known hypersensitivity to alpha2-delta (α2δ) ligands or other components of the study medications. Note: Prior exposure to DS-5565 is allowed, as long as hypersensitivity to DS-5565 was not observed.
• Subjects who are unlikely to comply with the protocol (e.g. uncooperative attitude, inability to return for subsequent visits) and/or otherwise considered by the investigator to be unlikely to complete the study.
• Abnormal investigative tests (i.e. electrocardiograms [ECGs]) and laboratory values judged by the investigator to be clinically significant at screening, with particular focus on: a. Abnormal renal function defined as calculated creatinine clearance (CrCl) < 60 mL/min determined by the central laboratory using the modified Cockcroft-Gault equation; blood urea nitrogen> 1.5 × upper limit of normal (ULN); creatine kinase > 3.0 × ULN; serum creatinine > 1.6 mg/dL (> 141.4 μmol/L); b. Abnormal liver function defined as aspartate aminotransferase (AST) > 2.0 × ULN, alanine aminotransferase (ALT) > 2.0 × ULN; alkaline phosphatase > 1.5 × ULN; total bilirubin> 1.2 × ULN. If a subject has total bilirubin > 1.2 ULN, unconjugated and conjugated bilirubin fractions should be analyzed and only subjects documented to have Gilbert's syndrome may be enrolled.