For as long as veterinarians have been giving drugs to animals, it has been recognized that individual patients can respond to therapy in unpredictable ways. Most animals will respond as expected, but in some patients the drug will appear to be completely ineffective, and in others the drug will cause unacceptable side effects. While, in many patients, simple dose adjustments can be used to address the problem, sometimes a particular drug simply cannot be used in an individual patient. When dealing with serious diseases that require aggressive therapy, where a drug that is ineffective can lead to death from the undertreated disease, and a drug that is not tolerated can lead to death from adverse effects, there is a natural tendency for veterinariansto never want to again use a particular drug with which they have had an untoward experience. This is particularly true with the immunosuppressive agents, where the immune-mediated diseases being treated can often be severe and life-threatening, and thetherapeutic window that provides immunosuppression without adverse effects or excessive predisposition to infection is narrow. With these drugs, there is a tendency for veterinarians to play "musical medications," abandoning a drug that they have a badexperience with forever, and switching to a newer immunosuppressive agent, often one they have never used before. The reality, however, is that there is no single immunosuppressive agent that is perfect for treating every immune-mediated condition in every individual patient. Moreover, even in drugs that are effective and well-tolerated in an individual animal, there is no single dose rate that is suited to all patients.
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