N-nitrosamines (NAs) are classified by the International Agency for Research on Cancer (IARC) and the US Environmental Protection Agency (EPA) according to their degree of carcinogenic risk. In addition, management standards are suggested and regulated by the World Health Organization (WHO) and US Food and Drug Administration (FDA). Based on these control guidances, NAs present in pharmaceuticals are infinitesimal amounts. Recently, various analysis methods using LC or GC combined with MS analysis methods have been proposed to manage trace amounts of N-nitrosamine present in pharmaceuticals. However, we observed the formation of a huge amount of NDMA in situ reaction with high temperature applied during HS-GC-MS system for some drugs such as metformin, nizatidine and ranitidine. This can lead to overestimation NAs quantification results and low reproducibility. To prevent the artificial formation of NAs during HS-GC-MS analysis, we developed a method by adding a nitrite scavenger capable of removing nitrites and nitrates to the API and drugs. Among various nitration inhibitors such as ascorbic acid, caffeic acid, and cysteine were applied to prevent the artificial formation of NAs. Headspce GC-MS conditions were optimized by testing incubation temperature, incubation time, concentration of each scavenger reagent, spike volume, and sample volume. The established HS-GC-MS method will help in the accurate quantification of NAs in various APIs and pharmaceuticals to reduce the in-situ formation of NAs.