Yes, it is entirely possible to suddenly develop new allergies as an adult. It can be frustrating when hives return after finishing fexofenadine (Allegra). Antihistamines help manage symptoms, but they do not cure the underlying trigger. To find the root cause, I highly recommend keeping a daily diary of your food, activities, and environment, and scheduling an appointment with an allergist. They can perform specialized testing to identify specific triggers. While there is not always a quick 'permanent cure' for hives, identifying your triggers and working with a specialist on a personalized plan is the best way to find lasting relief.