Her dementia only got really bad a couple years prior to this point, and that’s a huge bottle of pills, easily enough to last a few years, especially if not taken regularly (which she almost certainly wasn’t).

She probably already had it before her dementia got really bad and BoJack never bothered to look through her things from the nursing home (because of course he wouldn’t), so when she saw/overheard that Holly was overweight (by the standards of others), she found them with her things and started putting them in coffee like she had her whole life.