I quit taking Singulair a week ago, which makes me 6 days smarter than the FDA. I pretty much concluded that it was what was making me mean, groggy and stupid.
It took about 3 days to fully wake up, which counts as corroborating evidence as far as I am concerned. I doubt there was long term damage in my case. Detailed anecdote to follow.
It will be interesting to know how common my story is. I suppose I might be a rare outlier, but you can see how it doesn't much look that way from my point of view. I have a wild guess about the big picture with this drug but I probably shouldn't go mouthing off about it. I will be watching this one closely, though.
Update: Here's the FDA page for reporting drug reactions. Also, this site seems to be ground zero for conversation among (ahem) dissatisfied Singulair users.