Prevention Study in Adult Patients Suffering From Migraine Headaches

Inclusion criteria: * Outpatient subjects aged 18 years or older. * Females of non-childbearing potential. If of child-bearing potential, is not lactating and has a negative pregnancy test 7 days prior to study treatment initiation and agrees to use one of the GlaxoSmithKline (GSK)-specified highly effective methods for avoiding pregnancy. * Subjects suffering from migraine headache with or without aura, according to 2004 IHS criteria 1.1 and 1.2.1. * Subject has had a history of migraine headache for at least one year, and the age of onset was prior to 50 years. * Subject has consistent migraine headache over time (i.e., incidence and severity). * Subject has had at least three migraine headache attacks per month during the 3 months prior to screening and maintains this requirement during the last 4 weeks of the baseline period * Subject has had at least four migraine headache days but less than 15 total headache days (migraine or non-migraine) per month during the 3 months prior to screening and maintains this requirement during the last 4 weeks of the baseline period. * Subject is able to distinguish migraine headache attacks as discrete from other headaches (i.e., tension-type headaches). * Subject has the ability to read, comprehend and legibly and reliably record information in paper and electronic format as required by the protocol. * Subject must be able to provide written informed consent prior to participation in the study. The contents and process of obtaining informed consent will be in accordance with all applicable regulatory requirements. Exclusion Criteria: * Subject has a history of ergotamine, triptan, opioid, and/or combination pain medication use on \>/=10 days per month on a regular basis for \>/= 3 months. * Subject has failed more than 2 adequate treatments of migraine prophylaxis -where failure is defined as a lack of efficacy with treatment duration of at least 8 weeks. * Subject has history of simple analgesic use on \>/=15 days per month for \>/=3months. * Subject is unable to discontinue prohibited medications during the 2-week screening period and throughout the duration of the study including beta-blockers, benzodiazepines, tricyclic antidepressants, calcium channel blockers, antiepileptic drugs, bupropion or serotonergic noradrenergic reuptake inhibitors (SNRIs). * Subjects who have taken gabapentin or pregabalin previously for the prophylactic treatment of migraine headache. Subjects who have taken gabapentin or pregabalin for treatment of conditions other than migraine are eligible provided, (1) their total exposure to gabapentin and pregabalin is less than 3 months during the preceding 12 months, and (2) the subject stopped taking gabapentin or pregabalin for at least 3 months prior to baseline. * Subject has a history of cluster headaches or basilar, ophthalmoplegic, hemiplegic, or transformed migraine headaches. * Subject has a current or past history of seizure disorder. * Subject has any of the following medical conditions, laboratory abnormalities or disorders: * Hepatic impairment defined as alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \>2x upper limit of normal (ULN) or alkaline phosphatase or bilirubin \>1.5x ULN * Chronic hepatitis B or C with a positive Hepatitis B surface antigen (HBsAg) or Hepatitis C Core Antigen Antibody (Hep C antibody) * Impaired renal function defined as either creatinine clearance \<60 mL/min (estimation of creatinine clearance by Cockroft and Gault Method) or renal dysfunction requiring hemodialysis * Corrected QT (QTc) interval \>/= 450 msec based on the average QTc value of triplicate electrocardiograms (ECGs) obtained by the central ECG reader over a brief recording period * QTc interval \>/= 480 msec for subjects with Bundle Branch Block based on the average QTc value of triplicate ECGs obtained by the central ECG reader over a brief recording period * Uncontrolled hypertension at screen or at time of randomization (sitting systolic blood pressure \[SBP\] \>160 mmHg and/or sitting diastolic blood pressure \[DBP\] \>90 mmHg) * Medical condition or disorder that would interfere with the action, absorption, distribution, metabolism, or excretion of GSK1838262, or, in the investigator's judgement: * Is considered to be clinically significant and may pose a safety concern, or, * Could interfere with the accurate assessment of safety or efficacy, or, * Could potentially affect a subject's safety or study outcome. * Subject meets criteria as defined by the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) for a major depressive episode or for active significant psychiatric disorders within the past year, including dementia, general anxiety disorder, psychotic disorders or bipolar disorder. * Subjects with a history of depression that is in remission, with or without antidepressant treatment, may participate, unless a stable antidepressant regimen includes a prohibited medication. * Antidepressant medication may not be changed or discontinued to meet entry criteria and must be stable for at least 3 months prior to screening. * Subject has a history of clinically significant drug or alcohol abuse as defined by DSM IV TR or is unable to refrain from substance abuse throughout the study. * Subject is currently participating in another clinical study in which the subject is, or will be exposed to an investigational or non-investigational drug or device. * Subject has participated in a clinical study in which the subject was exposed to an investigational or non investigational drug or device: * Within the preceding month for studies unrelated to the current illness (migraine headaches), or * Within the preceding 3 months for studies related to the current illness (migraine headaches). * Subjects who have taken botulinum toxin type A (Botox) within the past 6 months. * Subject has a history of an allergic reaction, or a medically significant adverse reaction to the investigational product or excipients, which, in the opinion of the investigator, makes a subject unsuitable for participation in the study. * Subject is felt to be at risk of non-compliance (e.g., for taking investigational product or for completing the electronic diary \[e-diary\]), in the investigator's opinion. * Subject is a pregnant or nursing woman.