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I have been allergic to animal dander my whole life, but as some cruel twist of fate, I am absolutely crazy about animals. Fortunately, non-sedating antihistamines seem to relieve all of my symptoms. Despite my allergies, I'm considering getting a dog. Is there any harm in using an antihistamine every day? And will it continue to work with constant exposure to an allergen (in this case, dog)?
The use of long-term antihistamines is not a major concern in terms of drug safety. If you tolerate it now, you should continue to tolerate it. With the older, first generation antihistamines (e.g., diphenhydramine) there can be some loss of effectiveness (sub-sensitivity or tolerance), and while this may also apply to the newer, second generation, non-sedating antihistamines, the evidence is not conclusive.

However, when a patient lives with an animal, their exposure to the allergen is more intense. As a result, they may develop symptoms that are not adequately controlled with any oral antihistamine. In fact, most severely animal-allergic people often require more than one daily medication, and this usually includes a nasal steroid and an eye drop. While use of these medications on a daily basis is quite safe, we do not have data on safety for long-term (e.g., 20+ years) of daily use.

On the other hand, sometimes living with an animal actually slightly increases a patient's tolerance to the allergen, so that they have are fewer symptoms.

Unfortunately, there is not way to predict which of these scenarios will play out for you, or any other individual to a specific dog. You can be more allergic to one dog than another dog. In addition, a brief exposure to a specific dog may not predict how you will react to that same dog a few weeks later.

Another concern is that by allowing your nose to tolerate the dog (by taking an antihistamine), you are inhaling more dog dander. This increases the possibility of developing allergic asthma, as the lungs have a lining that is very similar to that of the upper airway. While this progression from nasal allergies to asthma is most common in children, it does also occur in adults.

If you are interested in having a dog, consult your allergist and consider starting on a course of allergen immunotherapy (allergy injections) to reduce your sensitivity to dog dander. This is the only form of preventative and long-term control medical treatment available. In addition, a dog-allergen-minimizing home environment would be helpful. Presuming you would have an inside dog, this might include hard surface flooring, leather furniture, a bedroom that is off-limits to the dog, and use of HEPA devices (vacuums and air cleaners).