Is there medicine for an injured mystery snail?

This is going to depend on what is actually wrong with the snail.

If you are seeing signs of shell deterioration, this is being caused by either a lack of calcium in the water, or your PH is too low (acidic), or a combination of both. Make sure that your PH is not acidic, and that you have a source of calcium in the tank such as simply adding some cuttlebone to the tank.