What are the financial consequences not to declare a general practitioner ("médecin traitant") in France? I have a French Social Security as well as a French "mutuel".
If you don't have a “médecin traitant”, there is a financial penalty. Physicians are allowed to charge more (dépassement d'honoraire), the statutory health insurance will only offer reduced coverage and the mutuelle should not cover the difference.
The idea is that you should get a referral from your GP before going to other medical professionals (it's called parcours de soins coordonnés, see service-public.fr and ameli.fr). There are exceptions, though: family planning, addiction treatment, emergencies, etc.