my own experience here.

Neurontin...brain-dead brain fog! A lessening of the ongoing pain[too fogged out to know/care], but, no brain!
Topamax, well, got my mind back, but pain maintained a solid steady hi-tension-wire 'thrum'
FINALLY tried Keppra, another anti-epileptic/seizure med. It's worked for me, got the brain back...can talk and write in complete sentences. Have pain, but
lesser than topamax and can really TELL if I skip/miss one. Uppping the dose by one tab puts me back in that 'dead zone'. There mite/should be a 'stickie' about the different options for PN pain meds....there are at least 40-50+ of them. For me this was the lucky one? Each of us, have different med problems that make scribing meds more 'quirky', for want of a better word.

Hope this helps? Hugs - j