Hello Waaris, The optimal treatment for severe house dust mite allergy is long acting antihistamines like Allegra 180mg once daily plus flixonase or nasonex nasal spray 1-2 sprays/nostril twice daily. Montelukast 10mg can be added at night if there is asthma such as in your case. Montair LC is a combination of montelukast and Levocetirizine that can be given in severe dust allergy. Montelukast has side effects though nothing very serious (such as nausea, vomiting, ocasionally dizziness hence taken at night, drynes of mucosa) and the only serious one reported in develpment of Churg Strauss syndrome, a vasculitis, though yet to be proven. Levocetirizine is a very safe second generation antihistamine. Use of the nasal spray sometimes allows you to withdraw the montair LC. You have to avoid dust as much as possible. The long term treatment is immunotherapy (subcutaneous or sublingual drops) against house dust mite given over 3-5 years. This should only be given by a specialist trained in this therapy. I hope this helps. Best Wishes.