The colonic microbiome is essential in health and disease, and is highly dynamic during the first several years of life. Proton pump inhibitors (PPIs) and histamine-2 receptor antagonists (H2RAs) are widely used in children, but the effects of PPIs and H2RAs on the pediatric colonic microbiome are unknown. This study will determine whether acid suppression with these medications affects the microbiome of otherwise healthy children who are prescribed acid suppression for gastroesophageal reflux disease (GERD), and determine the duration and magnitude of microbiome changes.