Most people with EPIC also have a Medicare prescription drug plan. Medicare, as the primary payer, is billed first, and EPIC then bases your co-pay on what's left over after Medicare. If the drug is not covered by your Medicare drug plan, EPIC is billed as the primary payer. Starting on October 1st, this will change.
After this date, if your prescription is not on the list of drugs covered by your Medicare drug plan, your pharmacist will receive a message that EPIC will likewise not cover the drug. At this point, you'll have the option of either changing to a drug that is covered by your Medicare plan (after which EPIC can be billed as a secondary payer), or agreeing to appeal the decision of your Medicare plan (after which EPIC will cover a 90-day supply of your prescription to last through the appeal's process). If you lose your appeal to your Medicare plan, EPIC will then again cover the prescription as primary payer.
If within the last few months you've used EPIC as the primary payer for a drug not covered by your Medicare plan, you'll soon receive a letter from EPIC explaining these upcoming changes. You should show this letter to your pharmacist or doctor to see if a drug that is covered by your Medicare plan can be substituted for what you're currently taking. You may also look into, if possible, changing to a Medicare drug plan that covers all of your prescriptions.