I used to get fever blisters frequently, from the sun, from a fever or just because. Now when I feel like I am going to get one, I take L-Lysine, it is an amino acid that our bodies have, and for some reason a person needs more of this amino acid at times. I have not had a fever bliser in years because of the L-Lysine. I used to get them so bad as a child, teenager, and when I was in my 20's, that I used to cry they hurt so bad. Different dr.'s have tried different medications on me, nothing helped. You can get L-Lysine at any drug store, k-mart, walmartor pharmor, it is inexpensive and it works for me. Like I said I take one when I feel like a fever blister is starting, and then I continue to take one for several days. Sometimes I have taken 2 pills at a timemaybe twice a day, it really works for me. Yes, the sun does cause fever blisters, at least it did on me.