Pharmacies can make and fill prescriptions for some Drugs. Ask them. If you need a script, there is a doctor's office in Grand Case (between the airport road and the bridge) that can give a script. He's walk-in and charges 30 or 35 Euros. My wife used him to get a script. It's recommended to get there prior to opening so as to not have too long a wait.

There is an ER in Marigot, but, I believe, their starting charge is 300 Euros.