Sara, I also had muscle pain (and nerve pain) before starting the B12. I'm not sure how much CFS was involved with its worsening, but I suspect it was involved as a series of liver cleanses had reduced the pain drastically for a couple of years, and it started coming back and getting excruciating (to the point of nausea when I moved it wrong) along with the illness.
It took, I think, somewhere in the neighborhood of 10 days for the cycle of increased spasms to lessened spasms to hey, it isn't hurting that much any more. I've also been using an infrared lamp, but I'm pretty sure the adb12 played a big part in that. I go to an acupressurist when the pain gets horrible, and she dramatically reduces it, but I haven't been able to afford that since well before I went on the adb12. I also had an Atlas Profilax some months before I found out about this B12 thing, and it helped quite a lot. All this is to say that I'm pretty sure that the adb12 was the main player in my latest bout of spasm and relief.
It's clear from reading this thread that people's reactions vary a lot, so my time frame and even my reaction may not apply to you - but it might. Good luck with that. I'll be interested to hear how it works for you.