Vitamins and minerals are commodity items, and every manufacturer has access to the same ingredients. For that reason, researchers and scientists say paying more for a name brand won’t necessarily buy you better vitamins.

“When we measure levels of vitamins in the blood, we find the levels are the same whether the person was taking a generic brand or a name brand,” says Dr. Rimm, who has been studying the effects of vitamins for 20 years.

That said, don’t be too cheap. Purchase your vitamins from well-known retailers that do a brisk business and restock frequently, whether that’s Costco or Drugstore.com. Vitamins lose their potency over time and must be stored at, or below, room temperature. If bottles are sitting on a shelf in warm room or in direct sunlight, they may degrade even before their expiration date.

PRICE MAY NOT MEAN QUALITY While the Food and Drug Administration regulates vitamins as part of the nutritional supplement industry, it does not test them before they are put on the shelves. The F.D.A. places the responsibility on the manufacturer to ensure that its dietary supplement products are safe before they are marketed. All of which means that no matter what the price, quality is not assured.

ConsumerLab.com, a company based in White Plains that tests hundreds of vitamins each year, finds that 30 percent of multivitamins have a quality problem: the pills might have more or less of a stated ingredient, or they might not dissolve properly.

“We haven’t found any brand with a broad product line that makes every product well,” says Dr. Tod Cooperman, president of the company.

Taking exception to such assertions is the vitamin industry’s trade group, the Council for Responsible Nutrition. In response to questions, the council released a statement from Andrew Shao, a vice president for scientific and regulatory affairs.

Mr. Shao said that the F.D.A. allowed for “a reasonable amount of variation” — which he characterized as up to 15 percent more of an ingredient than the label might indicate. Mr. Shao said that manufacturers frequently add slightly more of an ingredient to ensure that the amount is at least at the level claimed on the label as the product nears the end of its shelf life.

In any case, ConsumerLab.com says it has found a few patterns that consumers may find helpful. Products sold by vitamin chains tend to be more reliable than drugstore brands, and Wal-Mart and Costco’s vitamin lines are usually worth considering. In a recent test of multivitamins, ConsumerLab.com found that Equate-Mature Multivitamin 50+ sold by Wal-Mart was just as good as the name brand Centrum Silver, but at less than a nickel a day is half the price.

Puritan’s Pride, a catalog and online retailer, also has very good prices, and Dr. Cooperman says that its products are generally good.

Curious consumers can subscribe to ConsumerLab.com for $30 a year and learn how other supplement brands fare in the lab’s tests.