Join Us

Most Women Treated for Breast Cancer Take Dietary Supplements, Many Not Aware of Interactions With Hormonal Therapy

Save as Favorite
Sign in to receive recommendations (Learn more)

Up to 80% of women treated for breast cancer take at least one dietary supplement, but many women are not aware that the supplements they’re taking may interact with hormonal therapy medicines, a study suggests.

The research was presented on Dec. 9, 2020, at the 2020 San Antonio Breast Cancer Symposium. Read the abstract of “Dietary supplement use in a healthy eating and exercise lifestyle intervention in breast cancer survivors: The lifestyle exercise and nutrition (LEAN) study.”

Listen to a Breastcancer.org Podcast episode with Maura Harrigan, RDN, lead researcher of the study.

About dietary supplements
About this study
What this means for you

About dietary supplements

Dietary supplements include vitamins, minerals, botanicals, amino acids, and herbal preparations. People take dietary supplements for all kinds of reasons and with many different expectations. Research suggests that about 70% of people who’ve been diagnosed with any type of cancer take at least one supplement.

But it’s important to know two things:

  • Dietary supplements are not regulated by the U.S. government. All prescription and over-the-counter medications sold in the United States are regulated by the U.S. Food and Drug Administration (FDA). But supplements are not classified as medications, so their safety and effectiveness don't have to be carefully tested, as medications must. In other words, all supplements basically are considered "safe" by the government until they are proven unsafe. Medicines, on the other hand, must be proven safe and effective before they can be sold. This lack of regulation also means that you have no guarantee that the supplement you're buying is pure — meaning that it contains only the ingredients on the label. There's also no guarantee that the supplement has the exact amount of nutrient or herb or botanical in it that the label says it has.
  • Some dietary supplements can interfere with breast cancer treatment and other prescription medicines. Supplements can't always be safely taken along with prescription medication. Some supplements can change the way medications and radiation work in your body and may make the treatments less effective. For example, red clover and St. John's wort may interfere with the way tamoxifen works in your body. Most pharmaceutical companies and supplement producers do not conduct research on how medications and supplements interact, so we just don't know all the risks of taking supplements during treatment. It's very important that you talk to your doctor about any supplements you're thinking about taking.

Back to top

About this study

The results reviewed here are part of a larger study, called the LEAN study, which included 151 women who had been treated for early-stage breast cancer. The overarching goal of the LEAN study was to see if a diabetes prevention program could help women who had completed breast cancer treatment lose weight.

Many women treated for breast cancer gain weight during treatment, which can increase the risk of the breast cancer coming back (recurrence).

“It’s this very effective healthy eating and exercise intervention for weight management,” Harrigan explained. “And as part of our data collection … we also designed a medication and supplement questionnaire with the study. So, we were asking women to report their supplement usage at baseline. And it just was eye opening, to be honest.”

The results found that 80% of the women were taking at least one supplement. Overall, the women listed 54 different supplements that they were taking.

Most of the women — 60% — were taking three or more supplements, and 24% were taking five or more supplements.

Vitamin D was the most common supplement, taken by more than 50% of the women.

Of the 54 different supplements taken, 33% had potential interactions with two types of hormonal therapy: tamoxifen and the aromatase inhibitors.

“We [reviewed] these supplements and cross-checked them against the treatments people are receiving — particularly the hormonal treatments, as we did here with the aromatase inhibitors and tamoxifen — and there are interactions that occur that people are unaware of,” Harrigan said. “So, that's why we're trying to bring this to everyone's attention. This matters, and it often flies under the radar of clinicians.”

The researchers also asked the women why they were taking the supplements. A number of women reported taking supplements for a very specific health reason, such as taking calcium and vitamin D to support bone health.

Still, most women reported taking supplements to prevent cancer recurrence, which goes against current standards of practice.

“It’s very clear from the American Cancer Society, World Cancer Research Fund, American Institute for Cancer Research, all these standards of practice [say]: Do not use dietary supplements to protect against cancer,” Harrigan said.

“That message is not getting to people,” she continued. “So, they’re taking it with good intentions. You have to acknowledge that they’re really trying, and they're doing this on their own, and they're just working with information that's out there, and a lot of that information is not evidence-based and [there is] a lot of misinformation. So, they're doing the best they can.”

Back to top

What this means for you

If you are currently being treated for breast cancer or have completed treatment and are considering taking dietary supplements, it’s very important that you talk to your doctor and possibly a registered dietitian who specializes in oncology or an oncology pharmacist about what you would like to take and discuss all the risks and benefits, including any interactions that may occur with medicines you are taking.

It’s important to know that just because something is naturally occurring doesn’t mean it’s safe. Many people believe that any food or supplement in its naturally occurring, unprocessed state is better or safer than something that is refined or manufactured. This is not necessarily true. Some of the most toxic substances in the world occur naturally. For example, poisonous mushrooms and poison oak or ivy are extremely toxic to people but are completely natural.

Whether something is natural or synthetic isn't the most important question to ask. The most important questions are:

  • Will it benefit my health?
  • Is it safe?
  • Will it interact with any other medications or treatments I am receiving?
  • Does it have consistent and accurate doses?
  • Is it free of contaminants?

It’s also important to know that most doctors and dietitians recommend getting the nutrients you need from food, not from supplements.

“The micronutrients that are in supplement form do not act the same way as micronutrients in whole foods,” Harrigan said. “Your body does not handle them in the same manner. In fact, the nutrients that are in whole foods are much more available to your body, and the body handles supplements like it's a medication. So, there's really no replacing the power of Mother Nature, who packages all her nutrients in a matrix of foodstuffs — with fiber, with phytonutrients, with probiotics, and anti-inflammatory agents.”

“Though I take supplements, I still believe the overall health benefits I am receiving from my nine-plus servings of fruits and vegetables and one to three servings of soy foods each day are even more important than the benefits of these supplements,” said Diana Dyer, M.S., a registered dietitian and member of the Breastcancer.org Professional Advisory Board. “If I were forced to choose only one approach, I would put my money on maximizing my diet for the largest potential benefit.”

For more information, visit the Breastcancer.org Dietary Supplements pages.

To talk with others about creative ways to get nutrients from foods, join the Breastcancer.org Discussion Board forum Recipe Swap for Healthy Living.

Back to top

Written by: Jamie DePolo, senior editor

Reviewed by: Brian Wojciechowski, M.D., medical adviser


Was this article helpful? Yes / No
Rn icon

Can we help guide you?

Create a profile for better recommendations



How does this work? Learn more
Are these recommendations helpful? Take a quick survey

Fy22oct sidebarad v02
Back to Top