It's part of the reason I advise anybody on anti-seizure meds, many of which, like Neurontin, are dealt with by the kidneys rather than being metabolized by the liver, to take vitamin B supplements, including seperate B-12, and NOT to take them around the time of the med doses.

The flushing action of the meds on the kidneys tends to make it harder for b-vitmains to stay in the body long enough to be absorbed, so it is best to take them in goodly amounts. I take approximately 50mg a day of most of the B's, 800mg folic acid, and an additional sublingual 2000mcg/day of methylcobalamin B12, which is particularly hard to absorb when there is any medication interference.