Complementary, Alternative, or Integrative medicine (human or veterinary) is not one particular modality or treatment. There are many treatment methods which fall under these classifications.
Ideally, there is only medicine which has been shown to be effective scientifically, and that which has not. Veterinarians tend to use more treatments that have not been proven effective by well controlled clinical trials than are used in human medicine for a variety of reasons. These reasons include research limitations-there is much less money for veterinary medical research than for human medical research-the wide variety of different species veterinarians treat, and the financial limitations of animal owners. Since veterinary critics of alternative medicine are often criticized for using treatments that are not completely proven while attacking other unproven therapies, I will define complementary, alternative or integrative medicine as treatments that may require drastic changes in what is known about science to even work (improbable or impossible mechanism of action), treatments that may possibly have an effect but have not ever been tested or shown to be effective, and treatments that may have been proven, but are often co-opted by unscrupulous practitioners to either give themselves more credibility or take the treatment and exaggerate or twist it into something which is different than it was.
Of course, there is a wide spectrum of alternative therapies which range from completely ridiculous and improbable treatments such as homeopathy or therapeutic touch, treatments like acupuncture or chiropractic which might possibly have a physiologic effect but have still not been proven, to things like physical therapy, nutrition, exercise and vitamin supplementation that are part of any 'definition of medicine but are often claimed or abused by "alternative" practitioners. Some red flags for unproven therapies can include claims that the effect cannot be measured by scientific methods, large amounts of testimonials and/or anecdotal evidence, appeals to antiquity ("acupuncture has been used for 3000 years") claims that the treatment can only be explained by quantum mechanics, or claims that their treatment is being suppressed by some sort of conspiracy by big pharma and organized medicine.
Some of the people who practice these types of unproven treatments honestly believe that they work, and some are just out to make as much money as possible from gullible patients. In either case, it is unethical for a veterinarian and unfair to our patients to subject them to treatments that probably or definitely do not work. It is equally unethical to charge for such services and to encourage delusions that the pet is benefiting from them. Sometimes such delusions can lead to great harm and unnecessary suffering. Since veterinary patients are not able to choose for themselves, veterinarians have an added responsibility to advocate for their patients and to discourage treatments that are unproven, ineffective and potentially harmful.