Quinine is primarily used in the treatment of malaria, particularly for cases caused by Plasmodium falciparum, which is resistant to other antimalarial drugs. It works by interfering with the parasite's ability to digest hemoglobin, ultimately killing the parasite and reducing the severity of the infection. Quinine is often administered in combination with other antimalarial agents to enhance its efficacy and reduce the risk of resistance development.

Beyond its antimalarial properties, quinine is also used to treat nocturnal leg cramps, although its use for this purpose has declined due to potential side effects. In some regions, it is added to tonic water, giving the beverage its characteristic bitter taste. Additionally, quinine has been explored for its potential anti-inflammatory and antipyretic effects, though these applications are less common and not as well-established in medical practice.