Acyclovir (ask your oncologist). I use coconut oil to brush my teeth and as a ...mouthwash. It is also good to sooth mouth sores. . Acyclovir is my best suggestion
Also, it is reported to help prevent Shingles.
I am predisposed to Herpie Simplex ( fever blisters), and this is the reason I was prescribed Acyclovir, from the beginning and still take it daily. Dosage says twice a day but mostly, I take it once a day. You would have to be inside my brain to understand my reasoning of dosage.😬

When taking Vit D3 be sure to take Vit K2-MK7

If it is a cold sore, acyclovir and a topical cortisone can really help. Ask your doc. Also, a mild mouthwash dilution of water and peroxide may help. If your sores are open, see your doctor ASAP as a bacterial infection can occur. Otherwise, Popsicles at the first sign.

I have never used it, but many have shared using Magic Mouthwash. I use Chlorohexadine.
Paula🌹

My doc right after my SCT/BMT gave me a prescription for chlorhexidine gluconate. It's an oral rinse specifically used to prevent gingivitis and infections. I used for about two months postpost-transplant.