For most medications you can do a water change before adding more medication but some medications add a bit each day and build the levels up. If you are adding the same amount each time you treat the tank then it's fine to do a water change and gravel clean just before adding another dose of medication.
If you do a water change before re-dosing, do a big (75-90%) water change, and gravel clean all the substrate if possible.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
It's also a good idea to wipe the inside of the glass down before adding another dose of medication.