I've personally taken 12,000mg (12 grams, forty 300mg capsules) and the only ill-effect for me was diarrhea and stomach cramps/spasms. Gabapentin has horrible bioavailability and tolerance issues, so the best thing to do once you notice your tolerance rising is just lower your dose for a week or so, or try to stop taking it completely for a day or two if that's possible.
Gabapentin's going to be more powerful when combined with weed (more trippy), alcohol (more social but can lead to problems), and opiates (heavy sedation), and I'm pretty sure any other types of 'popular' anxiety medicines, sedatives, analgesics, muscle relaxers, etc. will also effect the strength and effects of the gabapentin.
Lots of people recommend staggering your dose (taking one or two capsules, then one or two fifteen/twenty minutes later, and so on) to increase bioavailability, but personally after about an hour and a half of staggering doses every fifteen minutes (usually 4 or 5 capsules every fifteen minutes), I always just end up feeling the same way I would if I'd popped 10 capules and another 10 forty-five minutes later, but maybe you can give it a try to see if it works any better for you.
The only other recommendation I can think of is to take the gabapentin with a caffeinated soft drink, I really don't know if it helps at all but I've got into the habit of doing it after 15 capsules and a 24 oz. Dr Pepper pretty much had me floored for almost 6 hours a few months after I was first prescribed it (three years ago).
I can relate to the irritability and attitude/anger issues you have when you try to take less than your usual dose and I completely understand not wanting to lower the dose just for that reason, but with gabapentin, the more you take, the more you're going to end up having to take, and it happens very very quickly. It'd be better to have it taken care of before you you're needing to take 6000mg at one time in order to benefit from the drug at all, and it seems to be really hard to talk to doctors/psychiatrists about developing tolerance issues since (unless they're a close family doctor or just an excellent, understanding doctor) most of them relate tolerance issues with abuse. A friend of mine who I think may have benefitted more from gabapentin than I did ended up getting taken off of it altogether for asking if there was a chance he could move to the 400mg capsules, since his 300mg 3 times a day wasn't working for him anymore.
I feel lucky that I'm still on gabapentin since it's really the only medication I've ever taken that's helped with all of my anxiety problems as well as stabilized/enhanced my mood, so when I know that I'm taking more than I need to, I give myself a reality check and cut down on my doses so that I'm not digging myself a deeper hole. Not saying gabapentin is overwhelmingly addictive or extremely hard to withdrawal from, but I wouldn't want to have to take an upwards of 20 capsules a day just to feel what I consider 'normal'. If you have access to all of the gabapentin in the world, I guess that wouldn't be a problem, though. Hope some of this makes sense and helps you out.
