Here are some important facts about dietary supplements such as herbs and vitamins:

They may affect how well other medicines work in your body.

Herbs and some plant-based products may prevent medicines from doing what they are supposed to do. These medicines can be ones your doctor prescribes for you, or even ones you buy off the shelf at the store.

For example, the herb St. John's wort, which some people with cancer use for depression, may cause certain anticancer drugs not to work as well as they should. A pharmacist can warn you when your prescription drugs will cause a dangerous drug interaction, so you should be sure your pharmacist knows what supplements you are taking.

Herbal supplements can act like drugs in your body.

They may be harmful when taken by themselves, with other substances, or in large doses. For example, some studies have shown that kava, an herb that has been used to help with stress and anxiety, may cause liver damage.

Vitamins can also take strong action in your body.

For example, high doses of vitamins, even vitamin C, may affect how chemotherapy and radiation work. Too much of any vitamin is not safe--even in a healthy person.

Tell your doctor if you are taking any dietary supplements, no matter how safe you think they are. This is very important. Even though there are ads or claims that something has been used for years, they do not prove that it is safe or effective. It is still important to be careful.

Supplements do not have to be approved by the Federal Government before being sold to the public. Also, a prescription is not needed to buy them. Therefore, it's up to consumers to decide what is best for them.

Creation of the National Cancer Institute (NCI) was authorized by the U.S. Congress in the National Cancer Act of 1937. NCI has evolved into the world's pre-eminent cancer research organization. The National Cancer Institute coordinates the National Cancer Program, which conducts and supports research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer, rehabilitation from cancer, and the continuing care of cancer patients and the families of cancer patients.