Some drugs may move from legally requiring a prescription, to an over-the-counter drug. When this occurs, it will no longer be eligible for reimbursement under your benefit plan. Only drugs which legally require a prescription will be covered.
Your insurance company reserves the right to change the formulary. Therefore, there may be instances where a drug was eligible in the past, but is no longer covered under your benefit plan.
There may also be instances where a drug was not properly adjudicated in the past and was paid in error. This would require further investigation.