There are many different approaches to calculation of significance criteria that can be used in interim analyses, and thus might be used to terminate a trial before the complete data are collected. The investigators have chosen an intermediate approach, based on the O'Brien-Fleming procedure [33] and set the interim test criterion at 0.01, and the final test criterion at 0.04. Using a criterion for significance of 0.04 and a 2-tailed test, this study will require 100 subjects in each group, for a total of 200 subjects, to have power of 80% to yield a statistically significant result. This computation assumes that the difference in persistent headache pain-free proportions is 0.20, specifically, 0.30 versus 0.50 (the rate of persistent headache pain-free 24 hours after ED discharge in a previous similarly-dosed metoclopramide trial was 30% [30]). The clinical effect being sought is comparable to a number needed to treat of five, the largest number needed to treat believed important to discover for this self-limited disease process. The interim analysis will have adequate power (0.80) to discover a statistically significant difference (alpha=0.01) if the difference between the two groups is 30% (.60 versus .30).