What is the treatment for cold sores that is best and has fewest side effects?

Pills, creams: Taking antiviral pills like Acyclovir early during the 1st 3 days of your outbreak can decrease your symptoms and help you heal faster. There are creams/ointments over the counter and prescription that can help you feel better too.

Time: The most side effect free treatment is to do nothing. Cold sores go away by themselves in 7-10 days. Sometimes longer. Anything else is going to have some side effects, so you weigh the benefit (getting rid of it earlier) with the risk of medication.