I`ve been using L-Lysine 1000 mg from Solgar a few years now, and i believe it`s the number one L-Lysine brand, with evident quality. It prevents cold sore outbreaks. Started to use it when i read about a study which refers the imbalance of the aminoacids L-Lysine & L-Arginine as one of the main triggers of cold sore outbreaks (if the amount of L-Arginine is superior to L-Lysine because of an unbalanced diet). For those with very often outbreaks, it helps cutting with chocolate, caffeine, peanuts, nuts e other seeds with high amounts of L-Arginine. Nuts can have 5000 mg of L-Arginine ! A huge trigger for someone with frequent cold sore outbreaks.