As the name suggests, vitamin B12 deficiency is when there is not enough of the B12 vitamin in your body. This can cause a multitude of symptoms, including a cognitive decline. A diagnosis is B12 deficiency may be a reversible cause of confusion and behavior changes.
In other words, it’s possible that increasing your B12 level can improve or restore your memory and ability to think clearly. It can also resolve the other symptoms you may be experiencing such as fatigue and weakness.
https://www.verywellhealth.com/is-it-alzheimers-disease-or-vitamin-b12-deficiency-98738