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Lysine is a vitamin supplement indicated for use in cats infected with FHV (feline herpes virus).

The amino acid L-lysine limits in vitro replication of many viruses including feline herpesvirus type 1 (FHV-1). The antiviral mechanism is unknown, however coincident arginine restriction has proved important in both in vitro and in vivo studies. Cats are exquisitely sensitive to arginine deficiency. Therefore application of in vitro data has been cautious to bolus administration of lysine to experimentally-infected cats.

Once-daily oral administration of 400mg lysine to 14 cats latently infected with FHV-1 was associated with a significant reduction in basal viral shedding, compared with placebo-treated cats. Twice-daily oral administration of 500mg lysine to eight cats beginning 6 hours prior to and continuing 3 weeks following prior inoculation with FHV-1 reduced severity of conjunctivitis relative to placebo-treated cats. Lysine supplementation has also resulted in plasma concentrations of arginine declining significantly in cats throughout this study. Despite this, no clinical evidence of arginine deficiency or change in body weight was noted in any cats. Possible reasons for the plasma arginine decrease are increased utilisation of arginine in the urea cycle during the formation of urea from ammonia and increased arginine utilisation during immune activation in cats experiencing more serious FHV-1-related clinical signs[1].

Twice-daily administration of lysine in a tablet or capsule form to individual cats in multi-cat environments, particularly feline shelters, may be impractical, stimulate further viral reactivation through stress or facilitate transfer of infectious organisms among cats by operators[2]. Bolus administration of a known quantity of lysine on to the cat's usual diet may be more practical but is still time consuming and would rely upon complete ingestion twice daily to exert the same bolus effect as that produced by individual tablets or capsules. Dietary lysine supplementation of cats may provide a means of nutritional control of viral shedding. It has also been demonstrated that as much as 86 g lysine/kg diet may be fed without any recognised adverse effect[3].

It should be noted that concurrent infection with Calicivirus or Chlamydia felis will affect the clinical response of cats to lysine supplementation.

Concurrent use of Famvir has shown a good clinical response when combined with daily lysine supplementation.


  1. Maggs, DJ et al (2007) Effects of dietary lysine supplementation in cats with enzootic upper respiratory disease. J Feline Med Surg 9:97-108
  2. Drazenovich TL et al (2009) Effects of dietary lysine supplementation on upper respiratory and ocular disease and detection of infectious organisms in cats within an animal shelter. Am J Vet Res 70(11):1391-1400
  3. Rees TM & Lubinski JL (2008) Oral supplementation with L-lysine did not prevent upper respiratory infection in a shelter population of cats. J Feline Med Surg 10(5):510-513