I am not an AME but there has previously been a thread to the effect that gabapentin or neurontin is not compatible with a UK medical. I am afraid I do not know about the FAA
My understanding is that this is only for the duration of administration
I must say this is an unusual drug for mild pain and also for sciatica, and I would not consider this to be common in the UK. As such you may find the AME is rather more interested in the underlying disease. However if the sciatica is better and you are off the drug you should be OK
Good luck