Does anyone know why some people experience severe weight loss when using opiates?

I became addicted to percs (another story) and have since lost about 50 lbs...my appetite is still there but no matter what I ingest cannot gain any weight. I have tried everything - protein shakes, dairy, carbs etc. Every doctor and tests have come up with nothing - including answers.

Has anyone have any ideas on this...I sometimes suspect that it has to do something with malabsorption...along the lines of perhaps a "lazy intestine". Any suggestions appreciated as things are getting worse...at this point have been weaning down to finally quit...I am 60 yrs old and it gets tougher each time. Thanks.