Dextran-functionalized maghemite fluid (DexMF) has been tested to treat Ehrlich-solid-tumor-bearing mice, evidencing its potential use in mediating magnetohyperthermia in breast cancer treatment. However, although magnetic nanoparticles tend to accumulate in tumor tissues, part of the nanomaterial can reach the blood stream, and then the organism. The aim of this study was to investigate the acute systemic effects of the intratumoral injection of DexMF mediating magnetohyperthermia in the treatment of an advanced clinical Ehrlich-solid-tumor, assessed through histopathological analyses of liver, kidneys, heart and spleen, comet assay, micronucleus test, hemogram, and serum levels of bilirubin, aspartate aminotransferase, alanine aminotransferase, gamma glutamyl transferase, alkaline phosphatase, creatinine, and urea. The tumor's histopathology and morphometry were used to assess its aggressiveness and regression. DexMF mediating hyperthermia was effective in containing tumor aggressiveness and in inducing tumor regression, besides showing no toxic effects. Its physical characteristics also suggest that it is safe to use in other biomedical applications.