The effect of drugs introduced into the conjunctival sac in the form of solutions is modified by a number of factors. One of the most important of these is their immediate dilution by tears always present in the sac and by those which rapidly reach it in response to the stimulation afforded by any chemical irritant. A second factor is precipitation of the drug substances present in the tears or conjunctival secretion or its chemical union with such substances. This is especially important when large amounts of highly albuminous secretion are present, as may easily be seen by the white precipitate of silver albuminate which forms when silver nitrate is applied to lids covered with purulent secretion. Such combination, of course, renders most of the drug inactive.
A third factor is that of the reaction of tissues and tears and of the solutions employed. Lipschütz1
GIFFORD SR, SMITH RD. EFFECT OF REACTION ON OPHTHALMIC SOLUTIONS. Arch Ophthalmol. 1933;9(2):227–233. doi:10.1001/archopht.1933.00830010240005