Praise be to Allah.
You have to make sure about any food or drink that enters your body, for every body that is nourished on haraam things, the Fire has more right to it. So you have to be sure. If anything that Allaah has forbidden has been added to milk or anything else, then in that case it is not permissible for you to drink it. Fish liver, however, is halaal even if it was “dead meat”, and sheep skin is permissible if the animal was slaughtered properly according to sharee’ah or if it was tanned properly.