If you are not satisfied with the treatment you have received through the Spanish National Health Service you can either ask for the Libro de Hojas de Quejas y Reclamaciones (complaints book) at your medical centre, or write a letter to the Servicio de Atención al Usuario of the relevant hospital.
For the complaint to be dealt with by the authorities, the letter should at least contain the following elements.
All centres should have a complaints form. You must leave one copy with the health centre and one to a ‘registro general’ in your town. Ask you town hall or often can be presented at the local unemployment offices.They will stamp a copy and send third to the relevant Junta department.
a. Name and surname
b. Postal address
c. Telephone number
d. Describe the act you are complaining about, date, department that treated you etc.
e. Date of the complaint, town you are writing from
f. Signature
g. Health Centre, Medical Centre or other medical service concerned
You can also download Queja forms online, through the websites of the Junta for your region.
For an example HERE is a link to the Valencia website.