Migraine :: Gabapentin - Side Effects Long Term?

My husband has been suffering with unbearable headaches for 15 years. Over the last few months they have become so much worse to the point that he is in a ball, crying in pain, he can't see, can't walk, he was ranting during one attack that he was going to take all of the tablets he could find. He has no memory of saying these things when the headache has eased. He tells me the headache is there constantly but gets worse really quickly. He usually wakes up with the headache although they sometimes come on during the day. They are usually at the sides of his head above his eyes but swaps sides and sometimes goes down his neck. He gets them everyday for a week then has a few days to a week then they start again. He has had a number of GPs all of which have said he doesn't need to be referred to a specialist it is migraines as if it was anything 'serious' we'd have known by now ie nOt a brain tumour. He has had many preventative medicines but none have helped. His latest GP has prescribed gabapentin which was prior to his last and worst attack so far. When we explained what happened she merely raised the dosage and gave imigran for pain relief. I just wondered if anyone has been prescribed gabapentin long term for migraines and if there are any long term side effects as I've heard there are a number namely vitamin B 12 deficiency, pulmonary problems, long and short term memory loss.