Couple things: First, 23andMe is not medical advice and I don't know how they get away with supplying such advice. Second, all medications incur risk to your body, so taking something like a Herpes medication for years in the hope that it might prevent Alzheimer's is a bit insane. A doctor needs to prescribe it (in the US, anyways) so you would have to have their approval anyways.

My mother has dementia, so this issue hits home for me, btw. But I don't see myself taking some medication in order to try and prevent something I may never end up getting. Herpes medications are not vitamins, if you get what I mean.