Sulfadoxine (also spelled sulphadoxine) is an ultra-long-lasting sulfonamide used in combination with pyrimethamine to treat malaria.

It is also used to prevent malaria but due to high levels of sulphadoxine-pyrimethamine resistance, this use has become less common.

It is also used, usually in combination with other drugs, to treat or prevent various infections in livestock.^{[citation needed]}

Sulfadoxine competitively inhibits dihydropteroate synthase, interfering with folate synthesis.^{[citation needed]}