Gabapentin

Gabapentin is an anticonvulsant drug primarily developed to treat epilepsy, but now more commonly used for chronic pain. It is being used off-label for ME/CFS and fibromyalgia patients mostly to augment pain relief. Common brand names for gabapentin are Neurontin, Horizant, which is used for restless legs syndrome, and Gralise. Gabapentin is very similar to Lyrica (pregabalin).

Evidence
Gabapentin is one of the drugs suggested for use in people with ME/CFS to treat related pain; it is suggested in both the Canadian Consensus Criteria for ME/CFS and the International Consensus Criteria for ME. 

Side Effects
Many side effects have been reported.

More commonly ones reported are:
 * clumsiness
 * unsteadiness, and continuous, uncontrolled, back-and-forth, or rolling eye movements
 * weight gain

More commonly one reported in children are:
 * aggressive behavior or other behavior problems
 * anxiety
 * concentration problems and change in school performance
 * crying
 * depression
 * false sense of well-being
 * hyperactivity or increase in body movements
 * rapidly changing moods
 * reacting too quickly, too emotional, or overreacting
 * restlessness
 * suspiciousness or distrust

Less commonly ones reported are:
 * black, tarry stools
 * chest pain
 * chills
 * cough
 * depression, irritability, or other mood or mental changes
 * fever
 * loss of memory
 * pain or swelling in the arms or legs
 * painful or difficult urination
 * shortness of breath
 * sore throat
 * sores, ulcers, or white spots on the lips or in the mouth
 * swollen glands
 * unusual bleeding or bruising
 * unusual tiredness or weakness

Learn more

 * Gabapentin - drugs.com
 * What's the difference between Lyrica and Gabapentin? - drugs.com
 * Nov 9, 2016, Gabapentin Side Effects: The Dangers Of Off-Label Prescriptions’ Surprising Side Effects, by Samantha Olson
 * 2016, Gabapentin (Neurontin) for ME
 * June 07, 2004, When drugs are used off-label, by Daniel Costello