Acyclovir. Fever blisters or cold sores are caused by herpes simplex virus and is contagious. Should be treated with oral antivirals such as acyclovir, valacyclovir (valtrex), Famciclovir (famvir). Topicals such as abreva and Denavir (penciclovir) are much less effective. Best to take the oral meds right when you feel the prodrome (tingling) which is the sensation prior to appearance of the blister or lesion.
Abreva, zovirax (acyclovir) Topical include abreva and zovirax (acyclovir) to speed up healing. To treat the episode and decrease recurrences oral zovirax (acyclovir), and if recurrences are frequent or severe than prophylactic zovirax (acyclovir) orally. Natural supplements containing lysine can help but require 6 pills a day. One to two zovirax (acyclovir) can be very effective in suppressing new herpes lesions.