Emily Oster
covers an old topic that's still controversial:
Many medical studies show positive health effects from higher vitamin levels. The only problem? These studies often can't tease out the effect of the vitamins from the effect of other factors, such as generally healthy living. Studies that attempt to do this typically show no impact from vitamin use - or only a very tiny one on a small subset of people. The truth is that for most people, vitamin supplementation is simply a waste of time.
[Author note: The article avoids what I'd consider the main question: since these studies are not exactly new, why does my doctor still recommend a multivitamin every time I visit? And what other things does our society universally accept that might be pointless?]