Gabapentin is a powerful prescription medicine used as a painkiller to treat partial seizures, epilepsy, and Restless leg syndrome. You can take gabapentin at night before sleeping for better effect. Gabapentin is one of the most potent painkillers. It has some severe and mild impacts. clumsiness, unsteadiness, dizziness, drowsiness, and sleepiness are the common side effects of gabapentin. Heart Attack is the harsh side effect of Gabapentin.