I have to use antihistamines regularly - usually fexofenadine, but today I used OTC antihistamine eye drops. The package insert said the drops were for "pollen, ragweed, grass, and animal hair/dander." I've noticed that most (all?) OTC antihistamines have similar lists of allergens for which you can take the drug. WRT antihistamines, does it really matter what the trigger was, once the reaction has started? If I'm having an allergic reaction to dust mites, and I take an antihistamine that does not list dust mites, it seems to work just fine.