Hydroxychloroquine, also known by its CAS number 118-42-3, is a pharmaceutical substance primarily used to treat malaria, rheumatoid arthritis, and lupus erythematosus. It belongs to the class of medications known as antimalarials and is derived from the closely related drug chloroquine. Hydroxychloroquine works by interfering with the growth and reproduction of parasites, thereby preventing the spread of malaria within the body. Additionally, it exhibits immunomodulatory properties that help control autoimmune diseases like rheumatoid arthritis and lupus erythematosus. This pharmaceutical substance is available in oral tablet or suspension form, providing a safe and effective treatment option for various medical conditions.