As a note. NEVER take out of date drugs. Yes they become way less useful. But they are like food. They do go bad. And just like food can cause problems. There is an expiration date on them for a reason. So always follow labels. That drug was way over a year old.
On a side note. Most likely nothing will happen. Maybe an upset tummy. And it will be ineffective.
Again please don't ever take out of date drugs. Your health is more important then dealing with a cold sore.
PS. I am NOT a doctor. I also have a huge fear of taking drugs of any kind. =) You have a lot more guts then I do. lol.