Horizant, the brandname form of gabapentin, is a prescription medication used to treat restless legs syndrome and nerve pain after a shingles outbreak, which is also known as postherpetic neuralgia. Horizant belongs to a group of drugs called anticonvulsants, which alter brain activity to change the way pain is sensed by the body. It is not known how Horizant works in treating restless legs syndrome. This medication comes in extendedrelease tablet form and is taken once or twice daily, with food. Common side effects of Horizant include sleepiness, dizziness, and headache. Do not drive or operate heavy machinery until.