I don't have MS , but I sure know the feeling!
I had horrible "foggy brain" in 03 & part of 04 from a combination of my repetitive work injury {RSI/TOS} and Lipitor
I really began to feel like I was getting Alzheimers and Attention deficit disorder!
luckily most of it resolved when I quit the Lipitor & got good PT & chiropractic to resolve the upper body muscles spasms & tightness.
I still can't multi task like I used to - one project at a time now LOL