Urine doesn't contain any toxins because it is not a waste. I think that while cleansing it is particularly useful to drink your urine, for two reasons:
1. when the body gets too acidic because of the die off, the urine you drink alkalizes it very fast. You can verify this very simply, the urine after tastes much less sour
2. drinking urine gives you a very direct feeling of the state of your body, so you can check quite easily what works fine for you and what doesn't.