Nifuroxazide-d4 is intended for use as an internal standard for the quantification of nifuroxazide by GC- or LC-MS. Nifuroxazide is a nitrofuran antibiotic. It is active against strains of the enteropathogenic bacteria C. jejuni, Salmonella, Y. enterocolitica, Shigella, and E. coli. It inhibits quorum sensing and virulence factor production in P. aeruginosa. Nifuroxazide inhibits STAT3 activity in a reporter assay and decreases viability of U266 and INA-6 myeloma cells, which have constitutive STAT3 phosphorylation, with EC50 values of approximately 4.5 µM for both. It also decreases viability, migration, and invasion of, and induces apoptosis in, MCF-7, 4T1, and MDA-MB-231 breast cancer cells. Nifuroxazide reduces tumor growth and prevents pulmonary metastasis in a 4T1 murine mammary carcinoma model. It also reduces diarrhea, weight loss, and colon inflammation in a rat model of acetic acid-induced ulcerative colitis.