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.