My 87 year old mother has burning mouth syndrome, and with that comes extreme dry mouth.
Before her last doctor retired, she was doing monthly B12 shots. The new doctor says her labs show they are not needed. Maybe so, but my
labs show normal levels too, but I requested shots anyway, after having similar issues, and my doctor agreed, fortunately.
So I finally convinced Mom to try oral B12 supplements,
I am happy to report that her mouth has improved significantly, after a couple months of a high dose. You cannot overdose on B12. Good luck.