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Hi Andrew,   This was a fascinating question and I ended up learning something entirely new, which is always fun. Although I know that asthma and GERD are closely related, I was surprised to hear that fexofenadine, the generic name for Allegra, was sometimes used with Cimetidine, AKA Tagamet, to treat allergy-related hives.   What I found out is that a condition called chronic urticaria, an allergic-like reaction of the skin to a usually harmless substance that is a result of an overreaction of an immune system substance called histamine, can be triggered by flushing of the skin, which would be likely when running. Hives are the symptom of this urticaria.   And chronic urticaria, which is what I assume you have, is treated with antihistamines, including fexofenadine. These antihistamines block H1 receptors in the immune system to control the allergic reaction. But that treatment is not always sufficient. In such cases, an H2 blocker such as cimetidine or rantidine may be added to treatment. The combination of the two types of immune system blockers may provide better relief.   Hope that helps, Kathi