DL and I butt heads on some issues related to use of drugs in show animals (actually only occasional use of tranquilizers) but I agree with her overlying concern on this issue. You have to rely on your vet and experienced teachers/advisors. It is pretty apparent from the volume of these types of posts that kids are getting little or no advice from adults who know what they are doing or think that drugs are silver bullets for cattle. For young people wanting to ask questions here concerning drug use, please post a little more background for your questions and you'll avoid some of the more terse responses or elicit more responses from others to help you. Information on drug labels is easily accessible via the internet. If you cant easily talk to a vet, look up the labels and you'll know first hand.
Also, selenium is actually fairly toxic to prople. Overuse of it could lead to some pretty serious problems and some residue issues.