Medicine is an art and a science. In medicine, given that we work with biological creatures, from time to time, outcomes are not what any of us would have hoped. Sometimes undesired outcomes will prompt an owner to question the treatment provided by their veterinarian.
If you are unhappy with an outcome, talk to your veterinarian. After all, if someone had a problem with something you'd done, you'd want them to talk to you about it instead of avoiding you and telling everyone else, right? Many times, what's perceived as an error is actually a failure of communication. The vast majority of situations can be resolved with an open discussion.
If you find that you have remaining concerns, a peer review board may be an option. Some state veterinary medical associations have peer review boards, and their role varies by state. To see if your state has a peer review board and to find out how it functions, contact your state veterinary medical association.
Lastly, if you continue to have significant concerns, you may submit a complaint to the state veterinary licensing board, which enforces state license laws applicable to veterinarians. This is a legal process that may result in disciplinary action against a veterinarian’s license or other remedies authorized by the license law.