Retrorsine is a naturally occurring pyrrolizidine alkaloid found in certain plants. However, due to its toxic effects on the liver and other organs, it is considered a dangerous substance. Retrorsine impurities refer to any impurities that may be present alongside retrorsine in a sample, which can impact the purity and quality of the substance. It is therefore important to carefully control and monitor retrorsine impurities to ensure safety and effectiveness in any application.