Nasal decongestants are used to relieve congestion and stuffiness in the nose caused by allergies, colds, and sinus infections. They work by narrowing the blood vessels in the nasal passages, making it easier to breathe. They come in various forms such as nasal sprays, drops, and oral medications. Nasal decongestants provide temporary relief and should not be used for more than a few days at a time without consulting a doctor.