How much sterile water alone, without medications or other dissolved substances, can be safely injected into the buttocks?

To be beneficial, fluids injected into the human body,
among other considerations, should be isotonic.  “Isotonic” means that the fluid has the
same concentration of salts as the body’s blood and cells.  Fluids with a greater
quantity of salt are called hypertonic, and those with less, hypotonic.  Sterile water
has the lowest possible content of salt (none) and occupies the low end of the spectrum
for hypotonic fluids.

There is no medical reason
to inject water into the buttocks.  Hypotonic injections are extremely painful.  The
water would cause immediate hemolysis (bursting) of the blood in the tissues, and lysis
(bursting of the cells) of the solid tissues as well.  This is because of the marked
difference of salt content.  If more than 10 or 20 ml were injected the patient would
end up with a seroma, which is a localized accumulation of degenerating blood, tissue,
and bodily fluids.  Although the seroma would eventually be reabsorbed and ultimately
heal, there would be a danger of infection and abscess
formation.

It occurs to me that you might be
considering injecting water into the buttocks for cosmetic purposes (a homemade “butt”
lift).  This would be painful, extremely dangerous, and would have no lasting effect on
the contour of the buttocks.  Actually, that’s not entirely true.  If the area becomes
infected, there may be scarring, retraction and permanent unsightly deformity of  one or
both buttocks.

The reference defines hypertonic
and hypotonic fluids.