An allergy is a sensitivity to a substance that is normally harmless. Flowers, trees, a family pet, or dust are examples of things that can cause an allergy. Even foods like peanut butter, fish, and milk can cause an allergic reaction in people with sensitivities to them. A substance that triggers an allergy is called an allergen. People can have different reactions to allergens, even when they are allergic to the same things. For example, an allergen that causes eye inflammation in one person may cause upset stomach in another. Compare to the active ingredient in Zyrtec tablets. One dose of Cetirizine Hydrochloride 10 mg tablets, works for a full 24 hours to relieve allergy symptoms. Cetirizine Hydrochloride Tablets are a antihistamine approved for the treatment of both indoor and outdoor allergy symptoms in adults and children 12 years of age and older. 24-hour temporary relief of these symptoms due to hay fever or other upper respiratory allergies: runny nose; sneezing; itchy, watery eyes; itching of the nose or throat.