Malarone tablets contain two active ingredients, proguanil hydrochloride and atovaquone. These are both medicines that are active against the parasite that causes malaria. Malaria is a potentially fatal disease caused by various types of parasites known as Plasmodium. Plasmodium are carried by mosquitoes and injected into the bloodstream during a bite from an infected mosquito. Once inside the body the parasite reproduces, resulting in the disease. Proguanil hydrochloride works by stopping the parasite from reproducing once it is in the bloodstream. It does this by blocking the action of a compound that is found in the Plasmodium parasite. This compound is an enzyme called dihydrofolate reductase, and is involved in the reproduction of the parasite. Dihydrofolate reductase normally converts folic acid into folinic acid in the parasite, which is a step essential for the parasite to produce new genetic material (DNA). New DNA is necessary for the parasite to reproduce. By blocking it's production, proguanil prevents malarial parasites in the blood from reproducing and increasing in number. Atovaquone also works by interfering with the production of substances needed by the malarial parasites to reproduce. It works in a slightly different way however, so these two medicines work in combination against the Plasmodium parasite. Proguanil with atovaquone is used to prevent malaria caused by a type of Plasmodium called Plasmodium falciparum. This parasite produces the most serious form of malaria (malignant malaria). Proguanil with atovaquone may be particularly useful for preventing malaria in travellers to areas where this parasite is known to be resistant to other antimalarial medicines, such as chloroquine.