2-Nitrophenol-13C6 is an impurity in the synthesis of labelled 4-Aminophenol (A618920). It is also a nephrotoxic metabolite of Acetaminophen (A161220) and Phenacetin (P294580) (1,2).
(1) Watanabe, et al.: Drug Metab. Dispos., 38, 1532 (2010)(2) Batuta, S. and Begum, N. A. Synth. Commun. 47, 137-147 (2017)