I don't think anyone really knows.

Gabapentin is excreted primarily whole by the kidneys. Some people seem to get fluid retention with this drug, so there may be a connection there.

I have seen studies that say, increases appetite. (that would be a CNS effect)

It could be that the sedation people have with this drug, makes them want to eat to feel better and more alert... sugar especially.

But you know the atypical antipsychotics cause weight gain and diabetes all by themselves. This may have to do with the pituitary because one of them, Risperdal has been shown to cause elevated estrogen, and also cause pituitary tumors not rarely either.

Complex drugs can have many effects on the body, and none of these new drugs are studied in depth alot by the drug companies. They just do the minimum to get on the market, and it may take 20 yrs of post marketing data collection to find these situations, and even fewer would have the financial incentive to find out "WHY".

So the real reason why some people gain the weight and others don't remains elusive and may only affect certain people with certain genetic makeups.

If someone can find links to the explanation of this side effect, I'd love to see it too!