Medicare drug plans usually cover Provigil, but not Nuvigil?

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I am on a Medicare prescription drug plan. I was prescribed Nuvigil by my sleep doctor, and he said that he was going to do a pre authorization. In the meantime, he gave me a starter voucher and savings program card to use for it. When I went to the pharmacy, they told me that my insurance doesn't cover Nuvigil. Though, my insurance does cover Provigil. I was surprised about that, cause I thought that more insurances tend to cover Nuvigil than Provigil, since Nuvigil costs less. I looked up the drug formularies of some other Medicare prescription plans and they all covered Provigil, but not Nuvigil.

I was wondering if anybody else here that is on a Medicare drug plan was able to get Nuvigil covered? Are some of these insurances wanting more people to be on Provigil instead of Nuvigil, since they can just switch a lot of the people to generic Provigil when it comes out? Is the generic Provigil still supposed to come out in 2012 in the US?