I took gabapentin for seven weeks at daily doses of 1200mg - 2100mg and it was by far the worst decision I've ever made in my life (which is saying something). It caused some major damage to my brain (which was already healing from being off benzos for six months) and my life has been a living hell ever since. I detoxed from a four year benzo use and what I've gone through with gabapentin made getting off benzos seem like a vacation at the playboy mansion.
I stopped gabapentin 17 months ago and suffer severe physical and psychological symptoms. They have improved slightly but it has left my life a complete living hell.
A lot of other people take it and don't have any problem with side effects or withdrawals. I appear to be in the minority. But to answer your question about it causing permanent brain damage, I would venture that the answer is: probably. Also, the people with issues with it appear to be people with current or previous dependencies with benzodiazepines, so keep that in mind.
I am not the only member who has gone through this. Another bluelighter, Toz, took gabapentin for one month and suffered withdrawal for two years before going back on it.
