Beware: if you are a doctor and one of your patients fails to pay their invoices, you risk breaching professional secrecy by filing a debt enforcement request against them.
Doctors are indeed bound by professional secrecy, which covers even the existence of the mandate. By filing a debt enforcement request against one of your patients, you disclose the information that this person was one of your patients. Such conduct may constitute a breach of professional secrecy within the meaning of Article 321 of the Criminal Code.
Beyond the criminal consequences of this offence, you also risk an administrative sanction that may go as far as a ban on practising. A breach of professional secrecy may indeed be regarded as an offence incompatible with the profession you practise.
Therefore, if you wish to recover your fees, you must first request release from professional secrecy from the competent authority. Once released, you may recover your invoices through enforcement proceedings.
Note that doctors are not the only ones bound by professional secrecy: lawyers, chiropractors, midwives, nurses and many other professions are also bound by it, as are their auxiliaries.
Every situation is different. If you are faced with this type of issue, I strongly encourage you to consult a competent adviser.