According to RxList,

"The most commonly observed adverse events associated with the use of gabapentin in combination with other antiepileptic drugs, not seen at an equivalent frequency among placebo-treated patients, were somnolence, dizziness, ataxia, fatigue and nystagmus."

Somnolence = drowsiness

Other terms you may not recognize are defined when you click on the above link.

Also, here's the insert that comes with a prescription.

Best,
John