Ketamine is pretty much non toxic in cases of acute dosing (weekend benders etc). The lethal dose is 8000mg+. For chronic use (long term) though, it can produce some rather nasty side effects - well it's not ketamine that'e responsible, but it's metabolite, norketamine - like the previously mentioned severe abdominal pain. From what I've read there are two sources of the pain, either pain from norketamine severely irritating the lining of the bladder, or it causing spasm of the gall bladder. Regardless of where it comes from, the pain is excruciating, as in curling up in the foetal position & writhing on the floor in agony; the idea about the stomach/drip isn't the case as I only IM ket, yet I've had it happen a couple of times.
As to why some suffer and others don't, it's down to how fast each individual clears norketamine from the body. The half life varies from a few hours to several days in different people amd I'd suspect that it's the ones who take a long time to clear it that suffer from the pain due to having ket in contact with the bladder wall for several days at a time (explains why heavy users are more likely to experience the pain). If you do get the abdominal pain, it's your body telling you to stop taking any more and give it a rest for a few days. Unfortunately, taking more makes the pain go away as ketamine is a very efficient painkiller, but that's only a temporary respite as you will end up suffering when you stop. People who continue to use, even after experiencing the pain, are what I can descibe as incredibly stupid types who probably are of the 'bury your head in the sand & hope the problem goes away' mentality - it wont go away, only come back with it's big brothers to give you serious metaphorical kicking.
If you can't learn/understand the message when the pain occurs, I'd say your only option is to never touch ketamine again (but very few seem able to grasp this most simple of concepts)