restless legs syndrome
"At first I thought Sinemet was the greatest thing ever.  I took it for two years and began to suffer all sorts of odd side effects which I didn't at first associate with the drug.  But the worst was one that nearly killed me.  I didn't even realize it was happening at first because I was having brief periods of blackout.  I'd be walking and suddenly feel as though the earth had shifted under my feet.  It finally happened while I was driving.  I snapped out of it, fortunately without hitting anyone or anything, but didn't know where I was.  I stopped taking it and all the symptoms I'd experienced disappeared."