I was reading Nicholas Saunders' book where he addresses this topic:
"When MDMA is swallowed, it is digested in the stomach and enters the blood stream. From there, some of it reaches the brain, but any MDMA that passes through the kidneys is removed and ends up in the urine - two thirds is excreted unchanged while some 7% is 'metabolised' into MDA. Every 6 hours the amount remaining in the body is roughly halved, so that after 24 hours there will be only about 3% left.(38, chapter 9) Other psychoactive drugs are excreted in a similar way, and this enables them to be 'recycled': Siberian reindeer hunters who take fly agaric mushrooms to get high drink each other's urine in order to prolong the effect.(39) "