Acyclovir
"I'm from the uk and Zovirax cost £6 which is probably $9 for a small little tube, it's rubbish, felt tingle so went straight to the shop and bought Zovirax, been applying it every 4-5 hours, 3 days later and the cold sore has blistered at side of my mouth,  would of been better using tcp, it burns a bit but it does the job"