Some scientists believe there is not enough evidence to recommend for or against taking a daily multivitamin, because there isn't yet enough data from randomized controlled trials.
That's a reasonable but short-sighted point of view since it may never be possible to conduct randomized trials that are long enough to test the effects of multiple vitamins on risks of cancers, Alzheimer's disease, and other degenerative conditions.
Looking at all the evidence—from epidemiological studies on diet and health, to biochemical studies on the minute mechanisms of disease—the potential health benefits of taking a standard daily multivitamin appear to outweigh the potential risks for most people.
The good news is, you don't need an expensive "designer" supplement or a name-brand kind to reap health benefits. A standard store-brand multivitamin-multimineral supplement is fine. Look for one that contains RDA-level amounts and that also has the United States Pharmacopeia (USP) seal of approval on the label.
The amount of vitamin D in most such supplements is usually 400 IU, so you might want to look for one that contains 800–1,000 IU of vitamin D or get additional amounts from a separate vitamin D supplement as well. In fact, the desirable intake of vitamin D is shifting swiftly; many people may need 2,000 IU per day (or more) to get their blood levels of vitamin D into an adequate range; particularly if they have darker skin, spend winters at higher latitudes (such as the northern U.S.), or spend little time in the sun. For premenopausal women, a multivitamin that includes the RDA for iron is a good idea.
Knowledge about the optimal intakes of vitamins and minerals is not set in stone. So it will be important to continue researching the relationships between vitamins, minerals, and chronic disease, over decades.
This may mean more confusing news headlines along the way, as the science develops. Read these tips on how to be a savvy reader of news articles about vitamin supplements and nutrition.