I have found that taking mega doses of L-Lysine at the first signs of a fever blister will lessen its severity or if taken soon enough, will stop the outbreak. Normally, L-Lysine is taken once a day ( one pill, 500 mg).
Taking 4000 mg (4 pills in the morning and four in the evening) really works. This is not harmful and has actually been studied as a cure. This has been a Godsend for me since I discovered this cure. L-Lysine is an essential amino acid that promotes healing.