Prednisone in Cystic Fibrosis Pulmonary Exacerbations

Inclusion Criteria: 1. Diagnosis of CF by newborn screening or at least one clinical feature of CF, AND either (a) or (b) as follows: 2. A documented sweat chloride ≥ 60 mEq/L by quantitative pilocarpine iontophoresis 3. A genotype with two identifiable CF-causing mutations 4. Age \> 6 years old. 5. Acute pulmonary exacerbation treated with IV antibiotics as previously defined 10% relative drop in FEV1 from baseline at the time of exacerbation 6. Informed consent by patient or parent/legal guardian 7. Ability to reproducibly perform pulmonary function testing 8. Ability to comply with medication use including the ability to take capsules, study visits and study procedures as judged by the site investigator Exclusion Criteria: 1. A respiratory tract culture positive for Burkholderia cenocepacia in the 12 months prior to enrollment 2. A respiratory tract culture positive for Mycobacterium abscessus in the 12 months prior to enrollment 3. Treatment with IV or oral corticosteroids within 2 weeks of enrollment or from Day 0-Day 7 of the pulmonary exacerbation 4. Active allergic bronchopulmonary aspergillosis (ABPA) at the time of enrollment as determined by treating physician 5. Asthma related exacerbation at enrollment as defined by the treating physician based on clinically compatible symptoms (eg. wheeze) 6. History of avascular necrosis or pathologic bone fracture 7. Uncontrolled hypertension with end organ damage 8. Active gastrointestinal bleeding 9. Status post lung or other organ transplantation 10. Pregnancy 11. Lactose intolerance (contained in placebo) 12. On Lumacaftor-Ivacaftor (Orkambi) at the time of exacerbation 13. Investigational drug use within 30 days prior to enrollment visit 14. Physical findings that would compromise the safety of the subject or the quality of the study data as determined by site investigator