​​Gabapentin 400 mg is a prescription medicine that’s used to treat partial seizures in adults and children who are at least 3 years old. More commonly, it's prescribed for nerve pain that's caused by herpes-family viruses (for which it is approved by the U.S. Food and Drug Administration) or other causes for which it is prescribed "off-label." Gabapentin is in a class of drugs called anticonvulsants. The medicine works by calming excitement in the brain and changing the way your body senses pain. The FDA approved this medicine in 1993.