Since Minolast FX (Montelukast and Fexofenadine) is no longer providing relief, your sinusitis may have worsened due to a secondary bacterial infection or increased inflammation. You should not switch to another prescription medicine without a doctor's evaluation, as you may require antibiotics or a prescription nasal corticosteroid spray (like fluticasone) rather than just antihistamines. I highly recommend consulting your doctor or an ENT specialist for an accurate diagnosis. Meanwhile, you can safely relieve congestion by using saline nasal sprays or rinses and performing steam inhalation two to three times a day.