Twenty-eight pharmaceutical companies risk having their products withdrawn from sale in the UK because they have not complied with safety guidelines to prevent the theoretical risk of ``mad cow'' disease transmission, the British Department of Health warned Monday, Reuters Health reports. Because of concern over transmissible spongiform encephalopathies, holders of licenses to market medicines in the UK were required to prove by March 1 that they complied with European guidelines on the use of animal materials in medicines they manufacture. The department said responses with respect to 15,745 products have been received by the Medicines Control Agency (MCA) but, despite reminders, nothing has been heard from 28 companies with licenses for 816 products in the UK. The department declined to name the companies involved. There are some 23,000 licensed medicines on the market in the UK. Of these, some 5,000 are licenses for products imported from elsewhere in the European Union (news - web sites) and a further 600 are European licenses issued by the European Medicine Evaluation Agency in London. The remainder are licenses issued by the MCA.