Permissible limits for impurities during clinical development has always been a grey area. Existing guidelines, specifically ICH Q3A only formally apply to marketed products. How factors such as duration of clinical trials have never been effectively addressed. A new paper looks in depth at how limits for impurities can be effectively linked to duration. It is hoped this will have a significant positive impact, that durationally adjusted limits will significantly reduce unnecessary toxicity studies and also unnecessary material reprocessing.