Decongestant: A drug that shrinks the swollen membranes in the nose, making it easier for a person to breathe. Decongestants can be taken orally or as nasal spray. Decongestant nasal spray should not be used for more than 5 days without a physician's recommendation. Many decongestant nasal sprays cause a worsening of symptoms (a rebound effect) when they are taken for too long and then discontinued. Decongestants should not be used by people who have high blood pressure unless they are under a physician's supervision.