Parenteral diphenhydramine is commonly used as adjuvant therapy for acute migraine despite the fact that data supporting this practice do not exist. The investigators propose a randomized double blind study to test the hypothesis that 50mg of intravenous diphenhydramine, when added to standard migraine therapy, will result in a greater rate of sustained headache relief than standard migraine therapy alone. For this study, standard migraine therapy will be 10mg of intravenous metoclopramide. Sustained headache relief is defined as achieving a headache level of "mild" or "none" within two hours and maintaining a level of "mild" or "none" for 48 hours. Patients who present to the Montefiore emergency room (Bronx, NY) with an acute migraine will be approached for participation. They will be screened for medication contra-indications and non-migraine etiologies of headache. The study will be randomized. Assignment will be concealed. Participants and researchers will be blinded. Efficacy outcomes and adverse events will be assessed every half hour for two hours in the Emergency Department and by telephone 48 hours after medication administration. A sample size calculation, based on pilot data, revealed the need for 374 participants. An interim analysis will be performed after 200 participants have been enrolled with the goal of assessing for lack of conditional power.

Inclusion Criteria:

• Acute migraine headache
• Present to our emergency room in the Bronx, NY for treatment of migraine headache

Exclusion Criteria:

• Temperature > 100.3 F
• Pheochromocytoma
• Seizure disorder
• Parkinson's disease
• Use of monoamine oxidase (MAO) inhibitors
• Use of anti-rejection transplant medications
• Use of potassium supplements
• Use of pramlintide