I learned about Duralactin when volunteering at an animal shelter. The minute any cat started sneezing, we generally would start it on Duralactin and other supplements to keep any upper respiratory or possible herpes virus infections from spreading to other cats.
My cat is a herpes virus carrier. Sometimes her flare-ups are preceded by allergies.
Lysine blocks arginine, upon which the herpes virus feeds. When I observe my cat getting symptoms ("pink eye", weepy eye, scabby nose tip, sneezing, etc.), I immediately start giving her the Duralactin (I used to give her lysine powder in her moist food every day, but it doesn't mix well with the kind of food she is eating--ideally, you want to give lysine every day, not just during outbreaks).
Sometimes she will lick the Duralactin off my finger, sometimes I must open her mouth and put it on her tongue.
I feel it shortens the duration of outbreaks, and maybe lessens the intensity of symptoms.
Note: Done under vet. supervision--a treatment plan we set up. I also use other medicines, like antibiotic eye drops (Never give a herpes carrier any kind of steroid for anything!)....Coconut oil has therapeutic value, some of the fatty acids. My cat will lick that off my finger (She does not like fish oil....).
A combo of supplements, medicines works best!
I order 5 or 6 tubes during the cooler months to reduce risk of it overheating in the mail...Wouldn't want to be caught without my Duralactin!