I hope I can post this without its being considered useful practical advice.
The Buddhist Monastic Code II: The Khandhaka Rules Translated and Explained says (added emphasis is mine),
A bhikkhu’s basic medicinal support is puti-mutta-bhesajja, which
translates literally as “rancid urine medicine” (Mv.I.30.4). Strangely, none of the texts
define the term. The commentaries to the Khuddakap›tha, Ud›na, and Sutta Nip›ta
give an example of this sort of medicine—rancid urine with yellow myrobalan—but
without a formal definition to indicate the full range of the term. The Sub-commentary
to the Vinaya defines rancid urine as any sort of urine at all, citing as a parallel the Pali
expression pÒti-k›ya, decomposing body, which refers to any human body, living or
dead, “even one with golden skin.” However, it does not say whether rancid urine
medicine is the rancid urine itself or, as suggested by the example from the
commentaries, rancid urine in which medicinal fruits are pickled.
Because the texts are vague about this term, various oral traditions have developed
around it. In Sri Lanka, rancid urine medicine is interpreted as rancid cow’s urine, in
which different types of myrobalan are sometimes pickled. In Thailand, some
Communities interpret it as one’s own first urine in the morning, following the ancient
Indian tradition of using this urine as a tonic. (Modern scientists have discovered that
this urine contains a high level of melatonin.) Given the silence of the texts, the best
policy here is to follow the traditions of one’s own Community.
For snakebite: A medicine may be made of the “four great filthy things”: excrement,
urine, ashes, and clay (!). [...] None of the texts
mention this point, but an oral tradition in Thailand asserts that the excrement to be
used in this medicine should first be burnt in a fire.
For jaundice: Urine and yellow myrobalan are allowed, which the Commentary
defines as yellow myrobalan pickled in cow urine. This raises the question: If this were
the meaning of rancid urine medicine in the four supports, why would there be this
(For drinking poison): “I allow that water
mixed with excrement be drunk” (Mv.VI.14.6)
There are various discussions of how to use it and whether or not there's any reason it seems likely to be helpful, for example here. Someone suggested for example that the (non-Buddhist) Iberians in Roman times used it like toothpaste for bleeding gums.
Also this suggests why it's allowable -- it's not a luxury:
Anyone who makes use of
Leftovers for food,
Putrid urine as medicine,
The root of a tree as lodging,
And rags from the rubbish-heap as robes,
Is at home in any direction.