Everyone needs vitamins. Too little Vitamin C and you get scurvy. Not enough B1 and you come down with beriberi. But even a halfway balanced diet provides you with enough essential nutrients to avoid vitamin deficiency diseases, so scurvy and beriberi aren’t things for most of us to worry about. 
A more important question for us developed world types is, Can large doses of vitamins help prevent other chronic conditions, like cancer and heart disease? The New York Times says no:
The latest news came last week after researchers in the Women’s Health Initiative study tracked eight years of multivitamin use among more than 161,000 older women.
Despite earlier findings suggesting that multivitamins might lower the
risk for heart disease and certain cancers, the study, published in The
Archives of Internal Medicine, found no such benefit.Last year, a study that tracked almost 15,000 male physicians for a decade reported no differences in cancer or heart disease rates among those using vitamins E and C compared with those taking a placebo. And in October, a study of 35,000 men dashed hopes that high doses of vitamin E and selenium could lower the risk of prostate cancer.
….“I’m puzzled why the public in general ignores the results of well-done trials,” said Dr. Eric Klein, national study coordinator for the prostate cancer trial and chairman of the Cleveland Clinic’s Glickman Urological and Kidney Institute. “The public’s belief in the benefits of vitamins and nutrients is not supported by the available scientific data.”
Eating leafy greens is good for you, but apparently getting megadoses of the same vitamins in pill form doesn’t do squat. In fact, they even have some negative side effects. Bottom line: have a salad tonight and skip the multivitamins.
