Chloroquine is a pharmaceutical used in the treatment of malaria. It has largely replaced quinine for this purpose. It received limited use shortly after it's discovery because it was thought it was very toxic to humans. However, clinical trials in the 1940s showed it to be superior to its predecessor and it was commonly used to prevent the contraction of the disease.
Long term use can damage the retina, and regular eye examinations are recommended for those who are on the drug for a lengthy period of time. One of the other troublesome side effects is that it can cause severe itching, particularly in persons of African ancestry.
Unfortunately, many strains of malaria are becoming resistant to chloroquine, although it is still very effective agains other strains.