Edit: my reason for questioning this is in regards to lessening the concentration of medication currently in the tank
I'll second Melody's advice--water changes will not decrease the medication's effectiveness.
My understanding is this: Many medications are not very stable, once in the tank. They break down in water after about 12 to 24 hours. So, when you re-dose, you are simply bringing the med levels back up to the concentration of a single dose.
You'll never have the concentration of more than a single dose in your water, unless you put in two doses at the same time.
What is building up in your water are the various ingredients of the medication, now broken down. When you do a water change, you only remove these ingredients that are already ineffective, anyway.
So, doing a water before a new dose will not decrease the med's effectiveness, because the dose you put in the day before has already broken down.