Butylparaben has been found in small concentrations in human breast fat and breast tumors. A review by the National Toxicology Program concluded, "There is no evidence of demonstrable risk for the development of breast cancer caused by the use of paraben-containing underarm cosmetics." Animal studies have been inconclusive. Oral administration of butylparaben to eight-week-old rats has caused tumors such as thymic lymphoma, non-thymic lymphoid leukemia, and myeloid leukemia. Similar studies, however, have found no significant increase in tumors upon butylparaben administration. Butylparaben is not listed as a carcinogen by the International Agency for Research on Cancer.