Quinine is a medication used to treat malaria and babesiosis. This includes the treatment of malaria due to Plasmodium falciparum that is resistant to chloroquine when artesunate is not available. While sometimes used for nocturnal leg cramps, quinine is not recommended for this purpose due to the risk of serious side effects. It can be taken by mouth or intravenously. Malaria resistance to quinine occurs in certain areas of the world. Quinine is also used as an ingredient in tonic water and other beverages to impart a bitter taste.

Common side effects include headache, ringing in the ears, vision issues, and sweating. More severe side effects include deafness, low blood platelets, and an irregular heartbeat. Use can make one more prone to sunburn. While it is unclear if use during pregnancy carries potential for fetal harm, treating malaria during pregnancy with quinine when appropriate is still recommended. Quinine is an alkaloid, a naturally occurring chemical compound. It possesses a C₉H₇N quinoline functional group (pyridine fused to benzene).

Quinine was first isolated in 1820 from the bark of a cinchona tree, which is native to Peru, and its molecular formula was determined by Adolph Strecker in 1854. The class of chemical compounds to which it belongs is thus called the cinchona alkaloids. Bark extracts had been used to treat malaria since at least 1632 and it was introduced to Spain as early as 1636 by Jesuit missionaries returning from the New World. It is on the World Health Organization's List of Essential Medicines. Treatment of malaria with quinine marks the first known use of a chemical compound to treat an infectious disease.

As of 2006, quinine is no longer recommended by the World Health Organization (WHO) as a first-line treatment for malaria, because there are other substances that are equally effective with fewer side effects. They recommend that it be used only when artemisinins are not available.^[why?] Quinine is also used to treat lupus and arthritis.

Quinine was frequently prescribed as an off-label treatment for leg cramps at night, but this has become less common since 2010 due to a warning from the US Food and Drug Administration (FDA) that such practice is associated with life-threatening side effects. Quinine can also act as a competitive inhibitor of monoamine oxidase (MAO), an enzyme that removes neurotransmitters from the brain. As an MAO inhibitor, it has potential to serve as a treatment for individuals with psychological disorders, similar to antidepressants that inhibit MAO.

Quinine is a basic amine and is usually provided as a salt. Various existing preparations include the hydrochloride, dihydrochloride, sulfate, bisulfate and gluconate. In the United States, quinine sulfate is commercially available in 324 mg tablets under the brand name Qualaquin.

All quinine salts may be given orally or intravenously (IV); quinine gluconate may also be given intramuscularly (IM) or rectally (PR). The main problem with rectal administration is that the dose can be expelled before it is completely absorbed; in practice, this is corrected by giving a further half dose. No injectable preparation of quinine is licensed in the US; quinidine is used instead.

Quinine is a flavor component of tonic water and bitter lemon soft drinks. On the soda gun behind many bars, tonic water is designated by the letter "Q" representing quinine.