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Chloroquine and hydroxychloroquine have been used for the treatment of malaria for more than 60 years. Until recently approximately 300 million chloroquine malaria treatments were given annually. In total over 15 billion treatments have been taken. Chloroquine was also the antimalarial prophylaxis of choice for travellers to the tropics. They are also used at higher doses for the treatment of other diseases such as rheumatoid arthritis and lupus.

These well-established drugs have an excellent safety record and are generally well tolerated. Side effects are usually mild and include stomach upset and nausea, headache and temporary blurring of vision. Chloroquine, more than hydroxychloroquine, can sometimes cause itching. If too high a dose is taken, these drugs can be dangerous or even cause death. They may cause toxicity to the eyes and muscles if taken regularly for years, but this does not occur with short term use (such as three months for prevention against COVID-19).