A French pharmacy (pharmacie) is a reasonable first stop for basic antiparasitic products — flea collars, spot-on treatments and similar — which pharmacists can sell over the counter without a prescription, alongside the human medication a pharmacy normally stocks. This reflects a genuinely active veterinary pharmacy sector within French pharmacies, distinct from how some other countries handle pet medication.
That counter sale generally stops at antiparasitics and similarly low-risk products. Anything that functions as an actual treatment — antibiotics, pain management, chronic condition medication — requires a prescription from a vet, the same as it would almost anywhere else in Europe, and a French pharmacy won't substitute for that visit.
If your pet is on regular prescription medication from home, bring enough to cover the trip comfortably plus a few extra days, since matching an exact brand on short notice at a French pharmacy or vet isn't guaranteed, even for a common medication.