Hi, 14 years ago when my son was 12 years old he had a liver transplant. When he got a cold sore on his mouth the Hospital for Sick Kids in Toronto prescribed Zovirax ointment for him in which they said could also be used by myself and my husband as well. My question is, Now I have a cold sore just getting started and I was prescribed Valtrex which has worked for me as well as in the past, but this time the Dr. also prescribed Xerese cream instead of Zovirax. He said it is a better cream. It says on the leaflet not to use on someone who has had a transplant or who is immunocompromised. Why is this cream so different from Zovirax and why can my son who is now 27 years old not use this cream?