If you are not satisfied with your provider or treatment, you should first discuss it with him or her. It may be just the result of a misunderstanding that can be easily resolved. If not, you can file a complaint (‘klacht’).
As of 2017, all healthcare providers are required to have a formal complaints procedure, which includes access to a complaints officer that is free of charge and can help you to find a solution. This person will also inform you about the possibilities to file a formal complaint.
Healthcare providers are required to be a member of an independent dispute settlement body, that will issue a binding decision on a complaint filed with this body.
Healthcare providers should have published on their website details about what to do if you have a complaint and of which dispute settlement body they are a member, though it will be likely in Dutch.