My 70 yr old mom was diagnosed with dementia two years ago and since COVID-19 hit last year, it's only gotten worse. She was stuck inside a lot and couldn't socialize with her friends and neighbors at the retirement community she lives in. Recently, my brother noticed that she is not bathing herself and not cutting her fingernails or toe nails. He took her to her neurologist to see if her meds could be changed, but all they said was starting her on a B-12 injection regiment. They did say the shots should help a bit. However, she has long term care and we are looking at starting a claim so we can get her the care she needs, which my brother and I cannot provide, should the injections not work in the way that we want. I recently realized that my grandmother (my mom's mom), who is 95 only started needing help with bathing 3 years ago. Seems like my mom is on the same path as my grandmother, only experiencing her symptons sooner rather than later.
(word of caution...there was a post recently where somone said that the LTC claim was denied because mom lived in a facility and that was not "home" so depending on the policy be prepared to advocate that the Retirement Community where she is living IS her home)
The B-12 may only make the doctor feel better, and have no effect on your mom. It might have some effect IF it is necessary but it will not delay, cure or reverse damage done by dementia.