Eshani Nandita, Ph.D., Assistant Director of Bioanalytical Chemistry at Emery Pharma, presented this work at the 68th American Society for Mass Spectrometry (ASMS) Conference in June, 2020. Ranitidine, a H2 histamine receptor blocker, is extensively used for heartburn and other stomach acid-related diseases. Recently, detection of the probable human carcinogen, N-Nitrosodimethylamine (NDMA), as an impurity resulted in extensive recalls of ranitidine medicines. We played an instrumental role in this discovery and showed that ranitidine is an inherently unstable compound, spontaneously forming NDMA at higher temperatures. Thermal instability of ranitidine limits the use of headspace (HS)-GC-MS for NDMA quantification in ranitidine, as commonly employed for other drugs. Herein, we describe an UHPLC-ESI-MRM MS method for the quantification of NDMA in drug products and biological matrices. The method allows for quantification of trace, low nanogram levels of NDMA per sample and was applied to evaluate thermal and biological stability of ranitidine. Additionally, to demonstrate whether NDMA is formed upon ingestion of ranitidine, an incubation study using ranitidine in simulated gastric fluid (SGF, containing added NaNO2) was performed. Results indicated that there was a substantial increase in NDMA levels when incubated in SGF. The propensity of ranitidine to generate NDMA in SGF suggests that such accumulation is highly plausible in vivo upon ranitidine ingestion. Our work highlights the importance of stringent specifications for impurity analysis in pharmaceuticals. Ranitidine has been widely used across the world for many years; it remains to be seen if the plausible NDMA exposure due to this has resulted in any adverse health outcomes.