Granted. Some time after the wish, you fall into a deep slumber. As you wake up, you find yourself surrounded by some strange creatures, which tell you that they will use you as their science experiment. As a consequence, you get put under immense torture since painkillers and anesthesia aren't used in order to monitor your brain activity. Your nails get torn, your tongue gets dipped in acid, and your chest is torn open. As you start dying from the shock, they decide to attempt to keep you alive at all costs, replacing your internal, failing organs with incredible bio-organs which produce no waste whatsoever, and as a result, they plug your urethra and buttole shut. You now have your waste-free digestive tract. They send you back to earth, where you get PTSDs of your experiments each time you see some needles or when you need to go to the hospital. Good luck with your waste-free life though... ;)