Spoke to three pharmacists yesterday as my husband gets bit bad and really swells. They all recommended he start taking allergy tablets 3-4 days prior to leaving so as to have them in his system and then one every day while we are gone. We bought the non-drowsy type. Keep in mind this will not prevent the bites but will definitely make life easier after the bite. Even our 6 year old grandaughter will start taking this as she swells up bad as well. Hope this info will help someone else.