I would like to introduce you to references used in this community in this thread.
Guidelines;
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ANVISA: Guide on Controlling Nitrosamines in Active Pharmaceutical Ingredients and Medicines ver.2 English translated by google uploaded by @Naiffer_Host
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Health Canada: Guidance on nitrosamine impurities in medications
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Japan: Self-inspection on Risks of Contamination with Nitrosamines in Drugs
Additional information;