Nasonex^® (mometasone nasal spray), a steroid that is available by prescription, is commonly used to treat nasal allergies (known medically as allergic rhinitis) and nasal polyps. An example of nasal allergies is hay fever. As part of a class of drugs known as corticosteroids, Nasonex works by decreasing inflammation. Nasonex is not suitable for everyone. A few of the conditions you should let your healthcare provider know about before taking the medication include tuberculosis, glaucoma or cataracts, and any allergies you may have. As with any medication, there are possible side effects of Nasonex, such as a sore throat, headaches, and nosebleeds.