The mice studies showing positive effect of viagra on dementia are from 10 or more years ago. Viagra went generic around that time: 2012-2013.
So has big pharma sitting on this for 10 years, just because they can't make money off a generic drug against dementia/Alzheimer, while at the same time asking for billions of funding for drugs against tau proteins, which ultimately turned out not therapeutic for Alzheimer, which makes medical people question the whole theory that tau proteins are the cause of Alzheimer...