To compare the safety and tolerability of sodium oxybate given as a combination of an oral solution and oral tablets for 4 weeks in subjects with fibromyalgia (FM).

Inclusion Criteria:

• Subject is able to understand the written informed consent
• Subject is 18 years of age or older.
• Subject meets the ACR criteria for fibromyalgia
• Subject is willing to discontinue prohibited by the protocol
• Subject agrees to use only non-sedating over-the-counter (OTC)medication
• Subject is willing to abstain from the ingestion of alcohol for the duration of the trial.

Exclusion Criteria:

• Subject has protocol prohibited medical & psychiatric conditions that would exclude subject
• Subject has a current or past history of a substance use disorder including alcohol abuse
• Subject has a clinically significant history of seizure disorder either past or present
• Female subject who is pregnant, nursing or lactating.
• Subject is diagnosed with sleep apnea
• Subject is unable to discontinue protocol prohibited medications
• Subject is experiencing fatigue and/or drowsiness/sedation in association with intake of allowed medications.
• Subject has taken any investigational drug within 5 half-lives of the investigational drug prior to signing the informed consent form.
• Subject has an allergic reaction to GHB or sodium oxybate or any of its constituents (e.g. malic acid).
• Subject is on a sodium-restricted diet.
• Subject has abnormal liver function test or other abnormal lab values
• Subject has an occupation that requires variable shift work or routine night shifts or cannot routinely spend at least 6 hours per night in bed.
• Subject has any other problems that, in the investigator's opinion, would preclude the subject's participation and completion of this trial or compromise reliable representation of subjective symptoms.
• Subject has been randomized into a previous Xyrem (sodium oxybate) clinical trial.