Sirolimus is a widely used immunosuppressant drug that is used to prevent organ rejection in transplant patients. However, the drug is not always pure and may contain impurities that can affect its efficacy and safety. Sirolimus impurities can include degradation products, process-related impurities, and other unknown impurities. These impurities can affect the drug's stability, potency, and safety, making it important to monitor and control impurity levels during drug manufacturing and storage.