LOS ANGELES — If you are taking vitamin supplements to reduce your risk of heart disease or cancer, a government panel of health experts wants you to know that you’re probably wasting your money. In some cases, those vitamins may actually increase your risk of cancer.

The U.S. Preventive Services Task Force came to this conclusion Monday after reviewing dozens of studies, including many randomized clinical trials, considered the gold standard for medical research. The task force’s final recommendation was published online Monday in the Annals of Internal Medicine.

Nearly half of adults in the U.S. take at least one vitamin or mineral supplement on a regular basis, including the 32 percent of adults who take a multivitamin-multimineral. These pills are advertised as a way to promote general health. In some cases, manufacturers promote them as cancer fighters and heart protectors.

Studies in animals and in laboratory dishes suggest oxidative stress contributes to diseases like cancer and heart disease, two diseases that together account for nearly half of all deaths in the U.S. If so, there’s reason to believe that antioxidants — including beta-carotene, selenium, and vitamins A, C and E — could be useful as preventive medicines.

But when the task force examined the medical evidence on vitamins, it found “inadequate evidence” to support the claims vitamin and mineral supplements benefit healthy adults. Multivitamins, individual vitamins and minerals, and specifically beta-carotene and vitamin E all failed to show they could reduce the risk of heart disease or cancer in people with no nutritional deficiencies.

“Cardiovascular disease and cancer have a significant health impact in America, and we all want to find ways to prevent these diseases,” said Dr. Virginia Moyer, who heads the task force. But so far, she added, the medical evidence does not show taking vitamins is helpful in this regard.