"When you drink your own urine, you are putting more water back into your system, but you are also putting in the stuff your body is trying to get rid of. This is not bad for you at first, but if you drink no water, only your own urine for an extended period of time, the urea content of your urine will increase. It becomes difficult to urinate, painfull sometimes. But note, this is after drinking your own urine for days.
Its like using a dirty air filter. It works, but soon the filter will become more and more clogged with dust. Just like an air filter, the urine, which now must dispose of more and more waste in the same amount of water, becomes saturated. This is observable, as if you do drink your own urine over a few days it becomes darker in color. Similarly, the more water you drink, the lighter your urine is.
Astronauts on the Apollo 13 mission were stuck in space without power. For the last four days or so of their journey, they had run out of liquid water. They drank their own urine, which kept them from dehydration while they came home."