A healthy person's urine is about 95 percent water and sterile, so in the short term it's safe to drink and does replenish lost water. But the other 5 percent of urine comprises a diverse collection of waste products, including nitrogen, potassium, and calciumand too much of these can cause problems. When you drink your own pee, all the stuff that your kidneys had attempted to excrete comes right back into your stomach, and much of it ends up back in your kidneys. After several days of this, your urine will become highly concentrated with dangerous waste products, and drinking it can cause symptoms similar to those brought on by total kidney failure. At that point, you're doomed either wayfrom dehydration on the one hand or renal meltdown on the other. (Even if one could filter out most of the unwanted products in urine, the cycle would not be sustainable for long. In addition to what he or she pees out, the average human excretes about half a quart of water a day through sweating and exhaling.)
