Dear Najibullah, Antihistamines taken intermittently sometimes seem that they are not working or you are developing a tolerance/resistance to it. However, this may seem but is usually not the case. What happens is that your allergies are getting worse, and once a day cetirizine is insufficient to make a difference to the symptos. Avil injection is a short-acting but rapidly acting antihistamine with sedative properties and will NOT recommend this for long term use. You must get a blood test or specific IgE OR a skin prick test to find out what your allergies are. If it is house dust mite, avoidance measures should be in place but optimising medical treatment with long acting antihistmines like allegra and nasal corticosteroid sprays are recommended. Immunotherapy (subcutaneous or sublingual) are reserved for patients who do not respond on medical treatment. Thanks.