Coughing is an important defense mechanism that helps airways in the throat and chest in the clearing of mucus or foreign materials. A cough may be acute (lasting less than three weeks) or chronic (lasting greater than three weeks). Certain medical conditions can cause coughing. These include upper respiratory infections, sinusitis, rhinitis, asthma, or gastroesophageal reflux disease. Certain prescription medications may also cause coughing as a side effect. A cough may be productive or nonproductive, and should be treated accordingly. For a nonproductive cough, a cough suppressant medicine is used for treatment. Codeine is available in some states in limited amounts without a prescription, and is used primarily for nighttime coughing. Dextromethorphan is another over-the-counter cough suppressant found in many products. Benzonatate (Tessalon Perles) is available only with a prescription for the treatment of a nonproductive cough. Guaifenesin is an expectorant which is used for a productive cough. It works by thinning mucus to improve the process of coughing and clear mucus from the airway. If a cough is caused by smoking, emphysema, asthma, or chronic bronchitis, treatment should not be used because the cough is acting to keep the lungs clear and permit breathing.