Used primarily to prevent and treat malaria caused by Plasmodium parasites, especially P. vivax, P. ovale, and P. malariae. It is effective in regions where the parasite remains sensitive to the drug. Also used off-label in the treatment of certain autoimmune diseases such as rheumatoid arthritis and lupus erythematosus due to its immunomodulatory effects. Works by increasing the pH within intracellular vesicles, interfering with antigen processing and reducing inflammation. Has been investigated in viral infections due to its ability to inhibit endosomal acidification, which some viruses require for entry into host cells. Not effective against P. falciparum in most areas due to widespread resistance. Typically administered orally and remains in widespread use in endemic regions where resistance is not prevalent.