Gabapentin is a medication used to treat epilepsy, pain, and certain types of nerve pain. It is also prescribed for the treatment of certain types of headaches. Gabapentin works by blocking pain signals from the nerves to the brain. Gabapentin is also a mood stabilizer and may be used to treat manic episodes in people with bipolar disorder. The most common side effects are drowsiness, dizziness, unsteadiness, nausea, and loss of appetite.