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?]
Why does your doctor recommend a multi-vitamin every time he sees you? Maybe because he wants to see you again soon.
If you eat a varied diet, you get enough vitamins. If you notice any symptoms of deficiency, eat more of the foods that contain whatever you are deficient in.
Supplements are a con. They often include dangerous remedies or dangerous chemical additives as binders, colourants or whatever, to give them more pop.
It is unwise to take anything on a daily basis. Maybe your doctor is trying to desensitise you to taking something on a daily basis so that he can progressively addict you to more and more dangerous substances and thus ensure your continued dependence on him and more and more business from you as the side effects of all the things he has you one start to stack up into actual illnesses requiring oh more poisonous chronic medications and more visits to him.
Multi-vitamins, the gateway drug of choice among today's white collared (*ahem* coated) pusher.
Perhaps a better medical insurance scheme would be one where we pay the doctor only so long as we are well.