Fel-O-Lysine Powder, 8 oz gives cats a supplemental source of L-lysine, which is an amino acid that can be used to help treat Feline Herpes Virus (FHV). The L-lysine in Fel-O-Lysine pet meds is effective against the virus' respiratory and ocular symptoms. It also boosts immune system function, tissue repair, and antioxidant activity.