First off, I'm not a bulemic, so you don't need to lecture me at all. I'm curious because I was reading aimlessly on webmd and learned that using laxatives to lose weight doesn't work because your body still absorbs the calories from the food you're purging. All the weight loss is reputedly water weight. But my bulemic former roommate lost, like, fifty pounds this way. All that can't be water. So. What gives? Any body know the details here?

Your loosing muscle . It weights more than fat. If you have nothing to support or feed your muscles you will loose them and become weak.

Did your roommate gain weight after she stopped using laxatives??? I'm sure she did, when using laxatives constantly u don't give ur body a chance to digest the food normally which leads to malnutrition, i would advice u of not trying this method for many reasons, one of them is extreme constipation and the always need for laxatives to use the bathroom, the other main reason it could inhibit the growth of colon cancer cells...
FYI muscle weighs same as fat, 1lb of muscle weighs just the same as 1lb of fat, the difference in both is that muscle is a tidy and sleek looking organ, while fat can be awkward and not nice looking.

some of it would be weight yes but ur body soon realises what happening and begins to shut down so the weight loss stops and its actually muscle thats being lost
eventually if it keeps happening the organs begin to get damaged and then a whole lot of trouble happens for the body

interesting!