List of products in the National Drug Code with proprietary name . When inhaled by mouth, acetylcysteine is used to help thin and loosen mucus in the airways due to certain lung diseases (such as emphysema, bronchitis, cystic fibrosis, pneumonia). This effect helps you to clear the mucus from your lungs so that you can breathe easier. When taken by mouth, acetylcysteine is used to prevent liver damage from acetaminophen overdose.