If you are using a Part D prescription drug plan ...
During the open enrollment period, EVERY YEAR you should go the
Medicare.gov, click on the Drug Coverage tab, select Find Health & Drug Plans and follow the instructions.
You will be able to estimate your out-of-pocket cost for your current drugs for the entire year. You will be able to select various pharmacies and compare your out-of pocket costs. This way, you will be able to select the plan and pharmacy combination which has the lowest annual cost for you.
You can do this right now, if you want.
You should this every year because Part D premiums change, preferred pharmacies change, formularies change, prices change.