Mepolizumab injection is used along with other medications to prevent wheezing, difficulty breathing, chest tightness, and coughing caused by asthma in certain children 6 years of age and older and adults whose asthma is not controlled with their current asthma medication(s). Mepolizumab injection is also used along with other medications to treat chronic rhinosinusitis with nasal polyps (ongoing runny nose, sinus swelling or nasal congestion, with or without a reduced sense of smell or pain and pressure in the face) in adults whose symptoms are not controlled with other medications. It is also used to treat eosinophilic granulomatosis with polyangiitis (EGPA; a condition that involves asthma, high levels of white blood cells, and blood vessel swelling) in adults. Mepolizumab injection is also used to treat hypereosinophilic syndrome (HES; group of blood disorders that occurs with high levels of certain white blood cells) in adults and children 12 years and older who have had this condition for 6 months or longer. Mepolizumab injection is in a class of medications called monoclonal antibodies. It works by blocking the action of a certain natural substance in the body that causes the symptoms of asthma.