People With Cancer Should Be Wary of Taking Dietary Supplements


January 19, 2023 – Cancer dietitian Lisa Cianciotta often finds herself sitting across from a patient who suddenly fishes a bottle of antioxidant supplements from their bag and says, My friend told me this works really well,or I read on the internet that this is supposed to be really good for cancer.

Although taking an antioxidant pill sounds harmless, Cianciotta, a clinical dietitian who works with cancer patients at NewYork-Presbyterian Hospital in New York City, knows well that this popular dietary supplement can interfere with a patient’s radiation or chemotherapy.

But many patients with cancer believe these over-the-counter vitamins, minerals, or herbal remedies will help them, and most use at least one dietary supplement alongside their cancer treatment.

And that leaves Cianciotta with a delicate conversation ahead of her.

Drug-supplement interactions are complex, often varying by supplement, cancer, and treatment type, and can do more harm than good. Popular dietary supplements may, for instance, cancel the effects of a cancer treatment, making it less effective, or increase serious side effects, such as liver toxicity. But in other cases, supplementation, such as vitamin D for patients who lack the vitamin, may be beneficial, Cianciotta says.

These drug-supplement interactions can be hard to pinpoint, given that more than two-thirds of doctors don’t know their patients are using supplements.

Here’s what patients need to know about the potential risks of supplement use during treatment, and how oncologists can address this thorny, often poorly understood topic with patients.

The Complex Drug-Supplement Landscape

The list of dietary supplements and how they can interact with different treatments and cancer types is long and nuanced.

But certain supplements appear to affect cancer treatments regardless of other things and should be avoided. Any supplement that strongly alters the body’s levels of the protein cytochromes P450 is one example. This group of enzymes plays a key role in metabolizing drugs, including chemotherapy and immunotherapy agents.

Certain supplements – most notably St. John’s wort extract – may decrease or increase the activity of cytochrome P450, which can then affect the concentrations of anticancer drugs in the blood, says William Figg, PharmD, an associate director of the Center for Cancer Research at the National Cancer Institute in Bethesda, MD. Studies show, for instance, that this common herbal supplement can increase the activity of cytochrome P450, resulting in lower levels of cancer drugs.

Outside of drug metabolism, patients with hormone-related cancers, such as breast and prostate cancers, should steer clear of dietary supplements that can alter levels of testosterone or estrogen, Figg says. The evergreen shrub ashwagandha, for example, is marketed to reduce stress and fatigue, but can also increase testosterone levels – a potential problem for those with prostate cancer receiving androgen deprivation therapy, which lowers testosterone levels.

Many oncologists counsel patients against using antioxidant-based dietary supplements – particularly turmeric and green tea extract – while they have radiation therapy and certain chemotherapies. These therapies work by creating an abundance of highly reactive molecules called free radicals in tumor cells, which increase stress within these cells, ultimately killing them off. Antioxidants, in theory, can neutralize this effect, he says Skyler Johnson, MD, a radiation oncologist at the Huntsman Cancer Institute at the University of Utah, Salt Lake City. Some studies suggest that antioxidant supplements may lessen the effects of radiation and chemotherapy, although the evidence is mixed.

Some dietary supplements, including high-dose green tea extract and vitamin A, can cause kidney or liver toxicity, and “many cancer patients already have compromised kidney or liver function,” says Jun J. Mao, MD, chief of integrative medicine at Memorial Sloan Kettering Cancer Center in New York City. Even herbs that do not interfere with how well a cancer drug works, such as stevia, can increase treatment-related side effects, such as nausea and vomiting.

Another potential problem with dietary supplements: It’s almost impossible to know exactly what’s in them. For instance, just last year, the FDA sent nearly 50 warning letters to companies marketing dietary supplements. The issue is that federal regulations governing production are less strict for supplements than for medications. As a result, some supplements contain ingredients not listed on the label.

One historical example was the supplement PC-SPES, a mix of eight herbs, marketed to men with prostate cancer. The supplement was recalled in 2002 after certain batches were found to contain traces of prescription drugs, including diethylstilbestrol, ethinyl estradiol, warfarin, and alprazolam.

To further complicate matters, some dietary supplements can be helpful. Most patients with cancer “are malnourished and missing out on nutrients they could be getting from food,” says Cianciotta.

Patients are tested routinely for vitamin deficiencies and receive supplements as needed, she says. Vitamin D and folic acid are two of the most common deficiencies in this patient population. Vitamin D supplementation can improve outcomes in patients who have received a stem cell transplant by aiding engraftment and rebuilding the immune system, while folic acid supplementation can help to raise low red blood cell counts and hemoglobin levels.

Although she rarely sees vitamin toxicity, Cianciotta stresses that more is not always better and supplement use, even when it seems safe or warranted due to a deficiency, should be taken under supervision, and monitored carefully by the patient’s care team.

Bringing Supplement Use Into the Light

Too often, providers are unaware of a patient’s supplement use.

A core reason: Dietary supplements are often touted as natural, which many patients equate with safety, says Samantha Heller, a senior clinical nutritionist at New York University Langone Health in New York City.

That means patients may not know a supplement can act like a drug and interfere with their cancer treatment, and thus may not see the importance of telling their doctor.

Still, the promise of herbs, vitamins, and minerals can be alluring, and there are many reasons patients decide to participate. One major appeal: Dietary supplements can help some patients feel empowered.

“Cancer is a disease that takes away a lot of control from the individual. Taking supplements or herbs is a way to regain some sense of control,” says Mao.

The phenomenon can also be cultural, he says. Some people grow up taking herbs and supplements to stay healthy or combat health woes.

Pressure or advice from family or friends who may think they are helping a loved one with their dietary recommendations may play a role as well. Friends and family “cannot prescribe chemo, but they can buy herbs and supplements,” Mao says.

Patients seeking greater control over their health or who feel high levels of anxiety may be more likely to take suggestions from friends and family or more likely to believe false or misleading claims about the efficacy or safety of supplements, explains medical oncologist William Dahut, MD, chief scientific officer for the American Cancer Society.

Plus, social media often amplifies and normalizes this misinformation, notes Johnson. In a 2021 studies published in the Journal of the National Cancer Institutehe and colleagues found that one-third of the most popular articles on cancer treatment posted to social media in 2018 and 2019 contained false, inaccurate, or misleading information that was often harmful.

Some of the false claims centered on unproven, potentially unsafe herbal remedies, according to Johnson. These included “lung cancer can be cured with cannabis oil” and “golden-berries cure and prevent cancer.”

Given exaggerated claims of “cures,” some patients may keep their supplement use under the radar out of fear they will be judged or criticized.

“Clinicians should avoid making patients feel judged or telling people not to go online to do their own research,” Johnson says.

Guiding patients, instead, to accurate sources of online information may be one way to help patients feel empowered, he says. and the Memorial Sloan Kettering Cancer Center’s About Herbs database provide accessible and accurate information on dietary supplements and cancer treatment for both health care professionals and patients, he notes.

If a particular supplement is not safe during treatment, providers should be able to explain why, says Cianciotta. In a recent study, 80% of health care providers surveyed believed that interactions between herbals and medications could be problematic, but only 15% could explain why.

“Being able to explain why we are discouraging a particular supplement right now tends to be much better received than just telling a patient not to take something, because it is bad,” she says.

Another key is listening closely to patients to understand why they may be taking a particular supplement. Does the patient feel out of control? Is nausea a problem?

“Allowing patients to tell you why they are using a particular supplement will often reveal unmet needs or psychosocial challenges,” Mao says. This information can allow providers to suggest an evidence-based alternative, such as mindfulness meditation or acupuncture to manage stress.

And if a patient has received a dietary supplement from well-meaning family and friends?

“Simply telling a patient that a given supplement is useless or harmful could create family tension,” says Mao.

Instead, he recommends reframing the issue.

“We want to have a better understanding of how patients are tolerating chemo or immunotherapy before throwing other things on top of it. Let them know that now may just not be the right time to add a supplement to the mix,” Mao says.

The bottom line: “Patients want to play an active role in their own care, and we want to help them do that in a safe way,” he says.


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