I am quoting from the book "The golden fountain - The complete Guide to Urine Therapy".
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Gargling:
Indication - Throatache, toothache, parodontosis, other ailments of tongue and mouth
Urine should be kept in the mouth for twenty to thirty min or, when that seems too much, as long as possible. Gargling with urine is an effective way to fight gum problems or other mouth and tongue disorders. In addition, toothaches disappears quickly and teeth stay healthy. It can heal aphts quite fast. After gargling, spit the Urine out.
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Text does not mention anywhere about using old urine. If you want to try anyway, then start from small amount of urine.