Levamisole has been used in humans to treat parasitic worm infections, and has been studied in combination with other forms of chemotherapy for colon cancer, melanoma, and head and neck cancer. In some of the leukemic cell line studies, both levamisole and tetramisole showed similar effect. The drug was withdrawn from the US and Canadian markets in 2000 and 2003, respectively, due to the risk of serious side effects and the availability of more effective replacement medications. The key toxic effect of the drug is agranulocytosis, a severe depletion of white blood cells that leaves patients vulnerable to infection.
Currently, levamisole remains in veterinary use as a dewormer for livestock. The medication has also been increasingly used as an adulterant in cocaine sold in the US and Canada, resulting in serious side effects.