Hydroxychloroquine 400 is used to prevent and treat acute attacks of malaria in adults and children weighing more than 31 kg (68 lbs). It is also used to treat discoid lupus erythematosus (DLE; a chronic inflammatory condition of the skin) or systemic lupus erythematosus (SLE or lupus; an autoimmune disease in which the immune system attacks healthy parts of the body such as joints, skin, blood vessels, and organs) and rheumatoid arthritis. Hydroxychloroquine is in a class of drugs called antimalarials and is also an antirheumatic drug. It works by killing the organisms that cause malaria. Hydroxychloroquine may work to treat rheumatoid arthritis and systemic lupus erythematosus by decreasing the activity of the immune system.