According to studies, it does no good and may possibly cause harm. Despite the lack of supporting studies and the slight possibility of causing harm, the use of lysine (an amino acid) supplements to treat upper respiratory infections was recommended by over 90% of the veterinarians contacted by the researchers whose work is described below.
The Winn Feline Foundation reports that according to a systematic review of studies, “Based on the complete lack of scientific evidence for the efficacy of lysine supplementation, the researchers of this article recommend an immediate stop of lysine supplementation for cats.”
These are strong words. The researchers are totally against the use of lysine.
The reason the report calls for stopping lysine supplementation is that “In fact, results from two clinical trials with cats even suggest that excess dietary lysine may have an enhancing effect on FHV-1 replication.”
So why do some owners see results after using lysine supplements? Perhaps the cats would have improved anyway.