Antihistamines work by blocking the effects of histamine, the chemical responsible for many of the allergy symptoms you experience.When histamine is released, it binds to special sites called receptors on cells in your nose and throat, causing them to swell and leak fluid. This results in inflammation, nasal congestion, runny nose, sneezing, itching, and other symptoms.
Antihistamines block the effects of histamine by "coating" receptors, which prevents binding. This, in turn, prevents nasal allergy symptoms.
Histamine works quickly once released. By the time your symptoms appear, the histamine has already attached to cell receptors, and the allergic reaction is well under way. That's why you need to take antihistamines 2 to 5 hours before exposure to allergens. Or you need to take them on a regular basis.
Some antihistamines go to work 15 to 30 minutes after they're taken. They reach their peak effectiveness in 1 to 2 hours. If you take an antihistamine before you're exposed to your allergens, the allergic reaction can be stopped.