I have had so many urinary issues like that from gabapentin. It does wacky things. Even though I am on a very low dose. It's a strange drug. I find it actually does relieve the "burning" aspect of this so i take a light dose at night. Otherwise, i would prefer to not be taking it. I also used to get bad migraines and be hypersensitive to sun and it alleviated that. I am still hoping to get off it completely at some point. the memory recall and diziness is not a welcome aspect of it. As far as weight gain. Dropped actually about 45 pounds since I started it. but, i also eat somewhat low carb. My supplement regimen also plays a part in the weight loss.

Yes--
 

--taking gabapentin or pregabalin will get the kidneys working harder. The fluid retention side effect seems, at least anecdotally, to happen more often in those with lessened kidney function. DEFINITELY intake fluids when on these drugs, and move around to tolerance to promote fluid elimination.

As to whether these meds have an effect on metabolism, many of the central nervous system anti-epileptics, with the exception of Topomax, seem, again anecdotally, to have some "slowing" effect in some people. But I don't think there are any double blinded well controlled studies out there (at least that I'm aware of) comparing basal metabolic rate and thyroid function for people taking doses of these versus those not.

Makes sense i only take my gabapentin dose at night and at that time i am basically asleep so I don't know the issues with retention. Although when i wake up to go its a little more of an effort than during the day when i am not taking it. i make sure to drink a ton of water.

Fluid retention
 

I definitely had fluid retention with the Gabapentin. My feet swelled so much that I have black and blue marks under my toenails. I was taking 500 mg of Naproxen with that also and I quit taking that. I am starting to notice the swelling with Lyrica now too. My Pain Management doctor told me to get a diuretic from my GP but I really don't want to take that too. I have been trying to walk more. I walk my dogs now for about 45 minutes to an hour a day and also try to go for a walk by myself when the pain allows me to.

I had horrible fluid retention in my calves on an overnight flight many years ago. My calves were twice the size. I was told to put my feet up and drink water. At the time I thought drinking more water when I already had fluid retention was nuts but it worked. Walking is really good thing to do for it also. When this happened my legs hurt too badly to walk.

how much gabapentin were u taking?

Only 300 mg 3 times a day. I think it might have been a combination of the Gabapentin and Naprosyn, which originally was 500 mg twice a day. I cut down the Naprosyn to 500 mg once a day but even that caused me to swell so much. It did help me also when I was able to put my feet up, but I had to go to work every day too so I wasn't able to do that all the time. I think the Naprosyn did help the pain quite a bit so I wasn't happy when I had to stop taking it.