Should you be taking a multivitamin? It's a good idea for members of certain groups who have special nutritional requirements. Among them:

Women who are pregnant or trying to become pregnant,

People with gastrointestinal disorders or other nutrient-depleting illnesses such as diabetes or cancer,

Strict vegetarians -- unless they're knowledgeable about how to get all the necessary nutrients from food.

Those on restricted diets.

For others, popping a daily multivitamin pill is probably not necessary if they eat a healthful diet. That means getting plenty of fruits, vegetables, legumes, whole grains and low-fat dairy products, and modest amounts of fish and low-fat meat and poultry.

Indeed, obtaining nutrients from food rather than pills offers noteworthy advantages. Food contains beneficial fiber as well as phytochemicals that may not only help prevent disease but also interact usefully with one another. And consuming plenty of nutritious foods leaves less room in your diet for unhealthful choices.

However, research has shown that most Americans don't eat the recommended diet. As a result, modest nutritional shortages are common, especially after age 50 -- even among people who eat a healthful diet. These can include deficiencies of vitamins D and B12. Some evidence suggests that taking multivitamins can fill such potential gaps and might reduce the risk of developing certain diseases in the over-50 group, as well as in younger people who don't eat nutritiously.

Our past tests of multivitamins have found that you generally can rely on the quality of brand-name and store-brand products. Buying multivitamins at closeout and dollar stores is another matter. In 2004, we uncovered problems with other products in those stores, so we went back recently and tested their multivitamins. The results: Nearly half of the 18 brands we tested failed to contain the labeled amount of at least one nutrient. And several did not dissolve adequately, an indicator of whether the pills break down fast enough to be absorbed.

Although we did not assess enough lots of specific brands to rate them individually -- consumers might have a tough time locating specific dollar-store products anyway -- our overall conclusion is that buying multivitamins at such outlets is a gamble not worth taking.

But there are more ways to waste your money on multivitamins than just buying unreliable brands. For instance, so-called "megadoses" of vitamins have no proven value and can drive up cost. What's more, they can be harmful. Megadoses of vitamin C can cause gastrointestinal upset, diarrhea and other side effects. And a mere 200 percent of the government's current Daily Value of supplemental vitamin A in the form of retinol can increase the risk of birth defects and liver damage.

Also, such exotic extras as herbal ingredients, food extracts and such phytochemicals as lutein and lycopene -- while probably safe -- are almost surely unnecessary. Trace minerals, such as boron or nickel, are similarly of dubious value.

If you have multivitamin needs, stick with the basics. All that most people require is a pill containing these 18 nutrients: vitamins A, B1 (thiamin), B2 (riboflavin), B3 (niacin), B6, B12, C, D, E and K, and folate (folic acid); plus calcium, copper, magnesium, phosphorus, potassium, selenium and zinc.