Alison wrote: "Sometimes acyclovir is given to help prevent the infection from coming back. Herpes infection resistance to acyclovir and the other antiviral medications is relatively rare. If they become worse, check with your doctor. Ideally, you should always keep a tube of Zovirax cream with you so that you can treat your next cold sore as soon as you remember, then continue as before. The symptoms much quicker than your bodies natural defenses would be able to prevent its appearance. Acyclovir may reduce the severity and duration of a shingles attack if given soon after the onset of symptoms. "