migraine
"I've been taking gabapentin (400mg) for 7 months now.  I used to wake up with a headache everyday and have a migraine a few times per month.  Since I started taking gabapentin I have not had a migraine and only on rare occasion do I wake up with a headache.  Even when I do wake up with a headache, it's gone within an hour.  
The only side effect I had was drowsiness during the day but, over time, that has gone away."