The present invention relates to impurities of donepezil, or a pharmaceut ically acceptable salt thereof, which may be produced during synthesis and s torage of donepezil, or a pharmaceutically-acceptable salt thereof, and to m ethods of identifying such impurities.The invention also relates to the use of these impurities in analytical techniques and to product that contain cer tain low levels of these impurities in particular the following impurities a re disclosed: formula (I) and formula (II).
