Arghh! By way of explanation, "the formulary" is another way of saying "your insurance company's list of covered meds" or the "Pharmacy Benefit Manager (PMB)." Both of these can and do change regularly, even if you don't personally change either your company or your plan. The whole PMB issue keeps cropping up in Medicare and insurance legislation because it appears this is what is keeping the prices sky high.
I have learned (the hard way) to "read the fine print" every year when the new insurance plan documents for the Medicare supplement comes out, to see if they are changing anything as far as drug coverage. A couple years ago several of our routine meds were changed from Tier 1 (low copay, no questions) to Tier 3 (HIGH copay, prior approval required) and one was dropped completely. Our med costs went from about $1000/year to almost $4000 - needless to say I went looking for a new provider. At the same time they made major changes to coverage for out-of-area care, and I got hit with $3000 of Urgent care bills.
Now, each year, when out plan brochures are published, I spend several hours reading them carefully, checking coverage & copay changes. Then I go through the exercise of making sure our doctors and medications are still covered by the plan - this sometimes takes a day. I actually go one step farther and call the drug benefit plan people (not the mail order pharmacy) to ask specifically about coverage for my husband's and my meds. Usually I can get a link to the formulary so I can review it.
Some Medicare supplement companies have representatives who will help you through this exercise - just be sure you are talking to a service/medical rep and not a salesperson.