Yes, it is possible that taking an antihistamine for a long period of time can lead to a reduced effectiveness over time. In this case, it might be helpful to switch to a different type of antihistamine to see if that provides better relief for your allergy symptoms. Another option could be to combine the antihistamine with a nasal spray containing corticosteroids to help alleviate congestion and other symptoms. It's always a good idea to consult your pharmacist or healthcare provider for advice on the best treatment options for your allergies.