A new systematic review has appeared discussing one of the most widely and longest used supplements in small animal medicine, the amino acid lysine, which is used to prevent and treat upper respiratory infections in cats caused by Feline Herpesvirus. I’ve only looked at the evidence concerning this supplement once in the past, and here was my conclusion at the time:
Lysine is an amino acid which is hypothesized to be useful in the prevention and treatment of Feline Herpesvirus (FHV-1) infections. This virus is extremely common, and many cats will be exposed and become infected as kittens. Clinical symptoms include sneezing, nasal congestion, and conjunctivitis, and they range from mild and self-limiting to very severe. Most cats will get over the initial infection, but many remain chronically infected. With suppression of immune function from stress, medication, or disease, the virus can re-emerge and cause symptoms again. A small subset of cats may develop chronic, ongoing symptoms associated with this infection. Vaccination reduces the severity of symptoms but does not prevent infection.
Lysine is proposed to interfere with the replication of FHV-1 by blocking the uptake of another amino acid, arginine. There are theoretical concerns that lysine supplementation could make cats arginine deficient, but experimental studies suggest this is unlikely in practice. So it appears to be safe, but does it work?
Well, maybe. For once, numerous studies have been done, but there is no clear, consistent pattern of results. Some show that oral supplementation is ineffective and might even make infection worse (Drazenovich, 2009; Rees, 2008; Maggs, 2007). Others do seem to demonstrate some benefit (Maggs, 2003; Stiles, 2002). So while lysine supplementation appears to be safe and there is a plausible rationale for its use, no definitive conclusion about its efficacy is justified.
The new review is less optimistic than my earlier assessment:
Sebastiaan Bol, Evelien M. Bunnik. Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review. BMC Veterinary Research 2015, 11:284
Taking all results discussed in this systematic review together, we conclude that lysine supplementation does not have an inhibitory effect on FHV-1 replication in the cat. The scientific data do not support lysine supplementation or additional research with cats, as has been advocated by some…Based on the complete lack of scientific evidence for the efficacy of lysine supplementation, we recommend an immediate stop of lysine supplementation for cats. Lysine supplementation is not effective to prevent cats from becoming infected with FHV-1, it does not decrease the chance of developing clinical signs related to active FHV-1 infection, and it does not have a positive effect on the clinical course of its disease manifestations. In fact, results from two clinical trials with cats even suggest that excess dietary lysine may have an enhancing effect on FHV-1 replication. Positive findings, either for HHV-1 or FHV-1, were the result of poor study design and could not be replicated in well-controlled, larger studies. Furthermore, the proposed mechanism of action of lysine-arginine antagonism does not work in cats and its result, lowering arginine levels, would be highly undesirable.
The table below from the article illustrates the evidence evaluated in this review:
Interestingly, most systematic reviews are reluctant to make active recommendations even when the lack of evidence is quite clear. The general thinking is that one can only prove a therapy does work, one cannot prove it does not. Certainly, additional evidence can always appear that shifts the probabilistic conclusions of science in one direction or another. However, we must make practical decisions about the risks and benefits of treatments based on the evidence we have, and at a certain point we must be willing to decide that enough is enough, that adequate negative evidence has accumulated to merit rejecting a therapy. These authors clearly felt that point had been reached for lysine and FHV-1 infection.
The following table gives their reasoning for discontinuing the use of lysine. (Based on the lack of supporting evidence and my own negative clinical experience with it, I have not routinely recommended it for quite some time).