Pharmacie wanting money for unpaid 'complementaire' going back over 2 yrs

Hi all

Our pharmacie said we are liable to pay for the 35% on 3 separate prescriptions going back to 2011/2012 which came back as unpaid (according to them). To make it worse, I'm covered 100% for 3 out of 4 items on one of them because of an ALD. We were fully paid up at the time but have since changed our 'complementaire' and are in dispute with the one in question. Anyone else experienced this? Why would it take all this time to come to light? Any advice would be helpful.