pure Lysine in moderate dosage cannot harm your cat.
(we should discuss a MG per cat Pound dosage here)
any Lysine the cat can't use will simply pass through
it's digestive and circulatory system under utilized.
however the Danger of off label dosing Lysine are any
additives that may be in the supplement from processing.
so if you can find something 100% pure and unprocessed
your cat should be safe. ironically, over priced Feline
Lysine probably has more flavoring and additives in it,
than pure human Lysine supplements do.
if after a Month the congestion symptoms have not
subsided, you should probably stop using Lysine and
see the vet after all, if congestion still concerns you.
before running out and buying anything, wait a few days
for cooler heads to chime in as this discussion evolves.