A variety of conditions can cause nasal congestion. A deviated septum is the most common cause of nasal obstruction. Enlarged turbinates is the second most common cause, which is usually associated with a deviated septum. Other structural problems with the nose, such as small nostrils, weak nostrils, scarring, and a narrow or collapsed nasal valve, may also contribute to nasal obstruction. Many other conditions, such as allergies, irritants, chronic sinusitis, medication side effects, and systemic illnesses, can contribute to nasal breathing problems. These conditions can be treated with medicine and may not require surgery.