Nasal polyps (singular: nasal polyp) are fleshy swellings, or polypoidal masses that develop in the lining of the nose and paranasal sinuses (air-filled spaces, communicating with the nasal cavity, within the bones of the skull and face). They are non-cancerous growths.olyps in just one side of the nose are not common. Large polyps and/or clusters can cause breathing difficulties. They can affect the patient's sense of smell. They may block the sinuses and cause frequent infections and other problems.