Ketamine for Acute Migraine in the Emergency Department

The investigators propose enrollment of patients upon or shortly after presentation to the emergency department for treatment with ketamine or placebo. 30 minutes after study drug is administered, patients will have the option to request rescue medication which will be ordered by the treating physician according to standard emergency department protocol. Pain scores will be documented using the Numerical rating scale (NRS-11), 4-Point Pain Intensity Categorical Scale, and 4-Point Functional Disability Scale at 0 minutes, 30 minutes, and 60 minutes post treatment. Follow-up telephone calls will be made 72-120 hours after treatment to assess the incidence of recurrence of head pain and patient satisfaction with the treatment. Subjects eligible for this study must present to the emergency department with a chief complaint of primary headache that is determined non-emergent by the treating physician. Upon block randomization, each subject will receive an intravenous dose of 0.2 mg/kg, with a maximum dose of 30mg, of ketamine or an equivalent volume of saline. After 30 minutes patients will be asked if they need rescue medication, and the treating physician will administer rescue treatment, if requested. The research hypotheses include: 1. The proportion of patients achieving significant pain reduction at 30 minutes will be higher in the ketamine treatment group compared to the placebo group. 2. The proportion of patients experiencing pain response, defined as \<50% reduction in the visual analog score compare to the baseline and a reduction of the 4-Point Pain Intensity Categorical Scale to a 1 or 0, will be higher for patients receiving ketamine compared to those receiving the placebo. 3. Patients treated with ketamine for headache will experience lower recurrence rates within 24 hours 4. Ketamine will demonstrate an adequate safety profile. The last two questions will be exploratory and not included in the power analysis. It is expected that ketamine will demonstrate a satisfactory safety profile with limited side effects. The proportion of patients attaining pain response is expected to be higher in the ketamine treatment group compared to placebo. It is also expected that patients treated with ketamine alone will be less likely to experience a recurring headache within 72 hours of treatment than those treated with placebo and rescue medication. The primary outcome will be achievement of pain response at 30 minutes after ketamine dose. Achievement of pain response will be defined as reduction in baseline pain score by at least 50% on the NRS scale. Secondary outcomes will include attainment of pain-free state, patient headache relief, recurrence of headache, recovery of functional disability, and need for rescue medication.