List of products in the National Drug Code with proprietary name . Baloxavir marboxil is an antiviral medication used to treat the flu (influenza) if your symptoms started less than 48 hours ago. It helps make the symptoms (such as stuffy nose, cough, sore throat, fever/chills, aches, tiredness) less severe and may shorten the recovery time by 1 to 2 days. Baloxavir marboxil works by stopping the flu virus from growing. It will not treat other kinds of infection besides the flu virus. Baloxavir marboxil is not a substitute for the flu vaccine. (See also Notes section.)