We just went through a very scary time when the formulary supplier of the only approved drug for my daughter stopped manufacturing the drug. The name brand (which she's used for 3 decades) is not on the formulary.
It is the only medication she which has consistently kept her disease in remission for that entire time. She had to have surgery which was minor - assuming her colon was not inflamed - but catastrophic if it was. It was not inflamed in October (after a year of doctors telling her she should just give up and have it removed - during 3-4 hospitalizations), but surgery couldn't be scheduled until early April - about 2 months after the discontinuation of the medication. She got special approval for a half-dose of the brand name. In order to have the surgery, she doubled up on the medication - hoping to get an exception or a manufacturer's coupon for post-surgery so she wouldn't have nothing post-surgery.
Different kind of formulary issue - since it was mid-year that it appeared, and we couldn't prepare for it. She finally got the supply from the manufacturer, but she has to pay more because it isn't on her formulary. (She's a $200,000 a year medical miracle - so she hits her out-of-pocket max in January. This blip adds payments on top of her out-of-pocket max.)
Staying healthy should not be this hard - especially when we have a proven track record of what is effective. But self-protection (getting your drugs through unconventional means, using up a prescription twice as fast as it supposedly should be used up, etc.) helps prevent disaster.