Urine is mostly sterile, and definitely safe to drink.
Most people forget that urine is mostly water. Water is the filter of the human body. Waste that our cells produce, collectivley known as urea, ends up in our bloodstream. Our kidneys remove the urea, along with excess water, and the result is urine.
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. I am not simply using the movie as the source of that info, they did in fact have to drink their own urine.
Doctors used to check for diabetes by tasting patients urine. If it was sweet it indicated high blood sugar. Luckily we have much more pleasant methods now.
The bottom line:
Urine is sterile. You can drink it. I don't know why you would want to, but it is completely safe.