I kissed someone with a coldsore, is there medicine to take to prevent the virus from effecting me?

When did this occur?: If it was within a few hours, perhaps you could get an antiviral oral medication (salt of Acyclovir like gancyclovir) which would interrupt this, but that is doubtful. Most people, probably including you, have already been exposed and have herpes simplex in their bodies already.

Herpes exposure: Yes. But, you should already know your herpes status. These new and accurate tests are a vital part of your medical history. 75% of americans have already been exposed to 'oral' herpes. The person you kissed could also be on a suppression treatment to make them less contagious. There is a lot to learn about herpes. Do not be afraid to ask your doctor about this.