Drink water. A lot of it. Add lemon, a lot of it. Also have a lot of b vitamin because it makes your pee yellow, since drinking a lot of fluids will make it crystal clear. For some strange reason they do not expect it to be clear.
This is old and everyone already said to pee a lot, but yeah! It would be scary to be in your situation, but I've never done drugs before so idk what that's like.