Lysine for cats – helpful or harmful?

If your cat has ever fallen victim to a upper respiratory infection, you may have heard of the common recommendation to supplement with lysine. It is thought that lysine, an essential amino acid, can shorten the duration and reduce viral replication of the viruses that cause conjunctivitis in cats – namely feline herpes virus and feline chlamydia (Chlamydophila felis). Cats may also have a number of viruses active at the time or be carriers which means when they experience a stressful event or a reduction in immunity, they may experience a flare up of clinical signs and develop a secondary bacterial infection requiring treatment from the vet.

Unfortunately, there is so much conflicting information on lysine and if its truly helpful, that I’ve collated everything I could find in this post in the hopes that it might help pet owners make an informed choice on deciding whether or not they would like to give it a try, if their cat suffers from recurrent infections.

So what is lysine?
Lysine is an essential amino acid. It must be present in the diet or supplemented as it is not synthesised in the body. Good sources of lysine are high-protein foods such as eggs, meat (specifically red meat, lamb, pork, and poultry), soy, beans and peas, and certain fish (such as cod and sardines).

What does it do and what is it used for?
Lysine has several roles in humans, most importantly proteinogenesis, but also in the crosslinking of collagen polypeptides, uptake of essential mineral nutrients, and in the production of carnitine, which is key in fatty acid metabolism.

In cats, L-lysine has shown some promise in the treatment and prevention of feline upper respiratory infections, particularly those induced by feline herpesvirus (FHV-1) and feline chlamydia, aswell as shortening the duration of infections and clinical signs. It is thought that the way it works is by lowering the arginine concentration in the body, but studies don’t support this as there was no change in arginine concentration even at very high doses of lysine.

What do the studies show?
Maggs et al (2003) showed that at 400mg per day for 30 days, fewer cats and eyes were affected by conjunctivitis, and onset of clinical signs of infection was delayed on average by 7 days in cats receiving L-lysine, compared with cats in the control group. Stiles, Townsend, Rogers & Krohne (2002) found that cats that received lysine had less severe conjunctivitis than cats that received placebo. Compared to Drazenovich et al (2009), who saw no significance in severity between the supplemented and control groups, and actually found that the supplemented group actually showed more severe disease (at week 4 of the trial). Other studies were unable to draw any significant conclusions on the efficacy of lysine and a recent literature review flatly denied any benefit of using lysine in cats. The main reason Bol & Bunnik (2015) caution against the use of lysine, is that it is believed to interfere with the viral replication by lowering the body’s concentrations of arginine, however Fascetti et al (2004) show that even excessive levels of lysine (800mg/kg) do not cause a relative arginine deficiency – nor did any other study I could find report an arginine deficiency. At the levels lysine is normally recommended at (400mg – 500mg per day), its unlikely to cause a problem, unless the supplementation is continued for extended periods of time.

Should I consider a lysine supplement?

If you want to give it a try, it won’t hurt. I recommend not using it for longer than a month at a time as this seems to be the accepted dosage. If your pet is experiencing an active infection, your vet may recommend a higher dose while the virus is active (500mg twice daily) and then drop down to a normal daily dose (500mg once daily) for suppression. Having used it with my own pets, I have seen it shown anecdotal benefits however using it in practice I haven’t seen it as effective which may be due to the fact its offered in clinic – as stress is a trigger for infection, do not supplement your cat if it is too stressful for them, as this will be counter-intuitive. Lysine is available in many forms (powders, pastes, flavoured liquids, oils, tablets and capsules), so if you are looking to try it out, make sure you consider the least stressful method of dosing for your pet in a flavour they will find appetizing to get the most benefit from it.