Medicine that Works...

I was at the drug store today to pick up some vitamin D. Living in the dark, cold north requires a bit of extra vitamin D. My levels are particularly low. The label said that any claims on the vitamins had not been evaluated by the FDA for efficacy in the treatment, prevention, or amelioration of any disease. Most of the vitamins on the shelf had doses that were ridiculously high. Being a relatively unregulated market, how does one determine whether the pills will do anything at all or worse, do something bad to you. Research has been accumulating that mega-dosing is hazardous (actually, haven't they been saying this for years?). Got me to wondering: if all the ineffective remedies were to suddenly disappear from store shelves, what would be left?