Sorry to post 2 times in a row, but, I am also one of those 'rare' cases who had a hideous reaction to Zofran (an SSRI type drug to stop nausea) and boy was this a doozy....movement disorder for days....and to make matters worse, the usual treatment is Benadryl, which does the same thing to me.

When I see some one not getting the prescribed effect from a drug, I wonder if they are not metabolizing or metabolizing too fast. That is when I look at what else they are one, to see if there is any one enzymatic pathway 'plugged' up, ala 'drug interaction'. More often than not, when it comes to pain treatment, an SSRI is involved.