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Hi I'm Dr Palmer, are you still there? It's amazing that person would even consider kissing anyone if what he had was a cold sore.
If the lip lesion was crusted or seeping fluid then it is far more contagious. This seems unlikely as usually these are painful and the person would not attempt to kiss someone. But even with a herpetic lesion that's not crusted, etc, it can still transmit the herpes virus. There are antiviral ointments you need a prescription for, but I would not worry too much. Symptoms usually take 1-2 weeks to appear. Just keep your lips hydrate, chap stick. Cracks in the lips/dry lips allow for easier transmission as well.
You could always ask that person too. If you know them. Don't be embarrassed, they should be embarrassed for kissing someone IF that was a herpetic lesion. Some people get them so often, they dont even care or dont even notice, especially if they are not painful.
If you start to notice in a week or so, that there is a tingling, or burning feeling on your lip, go right away to your doctor and get a script for Acyclovir or other antiviral ointment, because these are prodromal symptoms before a lesion/blister develops. This will allow you to eradicate it MUCH quicker. I hope I was able to help. Please rate me if you are satisfied and please ask me further Qs if needed and in the future as well.
Any further questions?