N-Nitroso-N-methylurea (NMU) is a highly reliable carcinogen, mutagen, and teratogen. NMU is an alkylating agent, and exhibits its toxicity by transferring its methyl group to nucleobases in nucleic acids, which can lead to AT:GC transition mutations.

NMU is the traditional precursor in the synthesis of diazomethane. However, since it is unstable at temperatures beyond 20 °C and shock-sensitive to a degree, for this purpose it has become obsolete and replaced by other (N-methyl)nitrosamides. Most chemical supply houses have stopped carrying it.

Acute exposure to NMU in humans can result in skin and eye irritation, headache, nausea, and vomiting.^[2] NMU is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity in experimental animals (IARC 1972, 1978, 1987).^[3] Various cancers induced in animal models include: squamous cell carcinomas of the forestomach, sarcomas and gliomas of the brain, adenocarcinomas of the pancreas, leukemia, and lymphomas.^[3] However, the actual potential for human exposure is quite limited, as the chemical is not produced or used in large quantities ^[3]

NMU is teratogenic and embryotoxic, resulting in craniofacial (cleft palate) and skeletal defects, increased fetal resorption, and fetal growth retardation.^[4][5][6] Exposure to NMU during pre-implantation, post-implantation, organogenesis, or by paternal exposure can result in these effects.