I've never had a cold sore, but I did get Epstein-Barr Virus (Glandular Fever) which then caused Myalgic Encephalomyelitis (ME aka Chronic Fatigue Syndrome [CFS]). Once you've had E-B it can reactivate whenever your immune system gets stressed. Lysine is said to make Epstein-Barr go dormant again, which is what I need! Taking this supplement has helped to give me a slight increase in energy/less fatigue. I happily accept this slight improvement, as any improvement is better than none when dealing with ME daily!