M.B.
The fever blisters are usually viral in origin. Have you tried Lysine? It is an essential amino acid and it is a fairly potent anti-viral:
http://www.umm.edu/altmed/articles/lysine-000312.htm
While that article suggests that we should be able to get the lysine we need from foods,
there is research that is showing that many foods no longer have the level of nutrients that they had in the past (depleted soil which impacts all food forms).
I have issues with alot of viruses - once you get some of these, they never leave you and stress is most often the trigger for the outbreaks. So, I take Lysine daily along with a range of amino acids. This helps tremendously and if I do get a cold sore due to too much stress, it clears quickly. I also found a lip gel that has lysine as the active ingredient - they're cleared in less than two days.
You also might look at your diet and see if it is maybe naturally low in lysine -
good sources of lysine are foods rich in protein, including meat (specifically red meat, pork, and poultry), cheese (particularly parmesan), certain fish (such as cod and sardines), nuts, eggs, soybeans (particularly tofu, isolated soy protein, and defatted soybean flour), spirulina, and fenugreek seed.
Also, arginine is another amino acid and it balances lysine, so if your diet is too high in arginine that could be depleting your lysine. Here's a list of some foods high in arginine:
http://www.nutritiondata.com/foods-000089000000000000000....
Lots of shellfish are very high in arginine and it's mostly high protein foods, which may well be why I need the extra lysine as I eat a high protein/low carb diet.