Used to treat malaria, particularly caused by Plasmodium falciparum. It is effective in cases where the parasite is resistant to chloroquine. Also used to relieve symptoms of nocturnal leg cramps, although this use is limited due to risk of side effects. In laboratory settings, quinine serves as a fluorescent standard due to its strong fluorescence under UV light. It is also used as a bittering agent in beverages, especially in tonic water, providing a characteristic bitter taste.