Hydroxychloroquine 400 mg dose (HCQ) is primarily used to treat malaria, lupus erythematosus, and rheumatoid arthritis. It works by interfering with the growth of parasites that cause malaria and by modulating the immune system in autoimmune conditions like lupus and rheumatoid arthritis, thereby reducing inflammation and symptoms. Hydroxychloroquine online has also been explored for its potential antiviral effects, particularly against SARS-CoV-2 during the COVID-19 pandemic, although its efficacy remains controversial and it is not recommended for this purpose outside of clinical trials. As with any medication, HCQ should be taken under medical supervision to manage potential side effects and ensure appropriate dosing.