Withdrawal. I was on Gabapentin for a month -- 2, 100 mg capsules at bedtime with the goal of waking up with no back pain (I have scoliosis which I treat with ibuprofen and yoga). I let the prescription run out and had two weeks of withdrawal symptoms -- fatigue, anxiety, insomnia and nausea -- it took me a week to figure out why I felt so awful, and by then I didn't want to go back and taper since I was through the worst of it.

I also take prozac 200 mg/day and I know that Gabapentin works in a similar fashion.