Reversible dementia may sound like snake oil medicine with little value. However, there are some of the many dementias that can be reversed because they do not leave permanent damage to the brain.
The following information is from the National Library of Medicine.
https://www.ncbi.nlm.nih.gov/
In the literature, potentially reversible conditions such as depression, adverse effects of medications, drug or alcohol abuse, space-occupying lesions, normal pressure hydrocephalus, and metabolic disorders such as hypothyroidism and vitamin B-12 deficiency are most commonly found in patients with cognitive impairment or dementia.
As you can see, there are situations in which outside influences or unrelated medical treatments can create the appearance of dementia. The apparent deterioration in mental abilities can be reversed if the unrelated problem, such as a nutritional problem, is corrected
If you are interested in reading about reversible dementias, I recommend the above website. The article contains a lot of medical terminology that you probably don’t want to look up, but there is information that is readable by lay people.
So far, I have not found any information that says that people with reversible dementia will return to 100% of their previous cognitive ability. If I find anything on this issue, I will update this post.