Nasal polyps are growths on the lining of the sinuses and nasal passages. Nasal polyps can develop when the mucous membrane of your nasal passages and sinuses are inflamed, typically for 12 weeks or more. Some of these growths can be large enough to block the nasal passages. When this occurs, it can lead to breathing problems, frequent sinus infections or loss of ability to smell. Nasal polyps are often associated with respiratory diseases such as allergic rhinitis, rhinosinusitis (chronic sinusitis), asthma or cystic fibrosis. Sensitivity to aspirin can also lead to the development of nasal polyps.