Antibiotic allergy: Antibiotic allergies can clear up in a day or last up to six months. Fortunately, most resolve relatively quickly, over a few days. We do not yet know why some persist for a long time and require strong medications like oral steroids (e.g. Prednisone) in order to settle down. ...Read more.our Allergies Can Be Treated Today! You are not alone. 50 million people in the United States suffer from allergy symptoms and its prevalence is increasing, affecting as many as 30 percent of adults and up to 40 percent of children.