The required withdrawal period is different for each medicine, and is determined through testing to see how long it takes for the medicine to leave the animal's system. This data forms part of the medicine's application for approval in each country, and will be scrutinised by the regulator. The withdrawal time period will then be stipulated on the product label. Back up checks are routinely carried out by the regulator to make sure no medicine residues are exceeded in meat or milk or other animal food products.