My BCBS Insurance Plan Formulary Lists Metformin SR 24 Hr

My BCBS insurance plan formulary lists metformin SR 24 hr on the internet documenting coverage for 2016. My doctor prescribed metformin ER. The Walgreens said it was not covered by the formulary. the BCBS phone support said SR is standard release not sustained release because their key to abbreviations lists ER for extended release and has no SR. However multiple web sources say SR stands for sustained release. No one discusses if in this drug the ER and SR are used interchangeably. My email