Fenistil cold sore cream is for the treatment of cold sores herpes labialis a cold sore is caused by a viral infection the virus can remain inactive for extended periods of time but when triggered the virus multiplies and a cold sore develops this can occur for example when you are run down when you have a cold or flu or when you have been out in strong sunlight the virus causes painful blistering of the lips how does it work fenistil cold sore cream contains the active ingredient p