After study eligibility was confirmed and all screening procedures completed, subjects were randomized at Visit 1 to receive either NXN-188 600 mg or placebo in a 1:1 ratio. Study drug and diaries were dispensed, and subjects were instructed regarding when to dose with study drug. Subjects were also trained regarding the Interactive Voice Response System (IVRS), and made familiar with the study diary and the completion of study diary assessments. Subjects left the clinic to self-administer treatment as an outpatient at the onset of a moderate to severe migraine headache without aura (as rated on a 4-point categorical scale). Dosing with study drug was to take place within 42 days from Randomization (Visit 1). The subject contacted the investigatorI or designee at 14 and 28 days after Randomization if they had not yet treated a migraine headache without aura in order to receive verbal permission from the site personnel to continue in the study as appropriate. If the subject had not treated a migraine headache within 42 days of Visit 1, the subject did not take study drug and returned all materials, including the unused study drug, to the study site. If the subject experienced a qualifying migraine without aura during the treatment period, (s)he recorded headache symptoms in the study diary when first noticed (and menstrual cycle status [female subjects only]). If the subject met dosing requirements as outlined in the protocol, they dosed with the study drug, called the IVRS to report dosing, recorded all assessments and adverse events (AEs) and contacted the study center to schedule a post treatment visit (Visit 2) within 6 days (± 2 days) of treatment. If study drug resulted in insufficient relief at 2 hours p.a., subjects were permitted to use the non-triptan rescue medication recommended by the PI at Visit 1. Within 6 days (± 2 days) of treatment, the subject returned to the study center for Visit 2. The subject had a brief physical examination, a 12-lead electrocardiogram (ECG), and had samples taken for clinical laboratory tests, including a serum pregnancy test for all females. Concomitant medications taken and adverse events (AEs) experienced from time of dosing to Visit 2 were reported to site personnel and recorded. Study diaries were reviewed and information recorded. Discontinuing subjects returned unused study drug and study material. One safety follow-up telephone contact was made within 15 days (± 2 days) post treatment. For those subjects not meeting the Treatment/Enrollment criteria by Day 42 (i.e., not experiencing a qualifying headache) and for other subjects discontinuing the study protocol post Randomization (Visit 1), a second visit, Visit 2, was required and subjects underwent a brief physical examination, had vital signs and ECGs performed, samples collected for clinical labs, concomitant medications and AEs reviewed, and returned all study drug and study materials.