After kissing someone with a cold sore, there is a chance of catching the virus, but in many cases no symptoms ever appear. Cold sores are caused by a very common herpes virus, and a single contact does not guarantee an infection. There is no proven medicine taken after one kiss that can reliably prevent infection, so doctors generally recommend watching for symptoms and treating only if they appear.
The virus spreads through direct contact with the sore or saliva, and many adults are already infected without knowing it. A first outbreak, when it happens, usually starts with tingling or burning on the lip, followed by small blisters that can be painful. These signs typically show up within about 2 to 12 days after contact.
During this time, it is helpful to watch for any tingling, burning, or new blisters around the mouth. If these appear, a doctor can recommend antiviral creams or pills, which work best when started early and can shorten the episode, even though they do not remove the virus from the body. Read more about cold sore medicine your doctor can recommend if you do notice a sore.