This is just a short post to draw attention to an excellent post by Edzard Ernst discussing the pitfalls of traditionally practiced herbal medicine. The only thing I would like to add is that the evidence for effectiveness and knowledge of potential toxicity is even less well known in animals than in humans. Especially in cats, which over the course of their evolution as obligate carnivores have lost some of the enzymes most other animals use to metabolize plant compounds.
As the research Dr. Ernst discusses demonstrates, traditional methods of individualized herbal treatment simply do not work, and should be avoided. The risks of toxicity, interaction and contamination are much higher than the basically random chance of benefit. This risk/benefit ratio is likely even more negative when treating animals, for which there is little to no strong evidence of efficacy, and potentially a higher risk, especially in cats of harm.