Hi and thank you so much for this query. I am so sorry to hear about this cold sore and other symptoms that you have reported. Penicillins do no cause cold sore but the infection for which you are taking penicillin is the most likely cause of this sore. I will suggest you discontinue valacyclovir as this is not needed for a single cold sore. If you have any further concerns about this sore, please get it examined by your doctor. Have you ever had cold sores before? I hope this helps. I wish you well. Feel free to ask for more information and clarification if need be and I will gladly respond to.