Impurities in drug substances and drug products: new approaches to quantification and qualification.

Regulatory requirements for the identification, qualification and control of impurities in drug substances and their formulated products are now being increasingly explicitly defined, particularly through the International Conference on Harmonisation. The implications of the recent guidelines are reviewed, both from their regulatory impact and the impact upon analytical technology. Impurities also have important safety consequences, and suggestions for possible routes to the qualification of impurities which do not involve the need to undertake additional studies are made.