Should Oncologists Continue to Be Ambivalent About Dietary Supplement Use by Their Patients?

Should Oncologists Continue to Be Ambivalent About Dietary Supplement Use by Their Patients?

Abdul-Rahman Jazieh, MD, MPH

Oct 24, 2023

After reconciling the short list of the prescribed medications, I asked my patient if she was taking any non-prescription medications or supplements. Calmly, she pulled out a large paper bag full of all kinds of substances, including vitamin C, shark cartilage, turmeric, multivitamins, mushroom products, etc.—a scene that is not unfamiliar to many oncologists.

Over the last 3 years or so, I adopted a clear position to advise my patients against the use of dietary supplements (DS), especially during cancer treatment. This position was an evolution in my approach over a couple of decades; I went through stages of indifference, curiosity, ambivalence, and caution, to full-fledged opposition.

Initially, I started to study the prevalence of DS use among different populations of patients with cancer and, like many investigators reported, there was a huge prevalence of DS use that was not disclosed to physicians routinely. The goal at that stage was to encourage physicians to actively inquire about DS use and for patients to fully disclose the use to their physicians.

Over the years, I realized that even when patients disclose the use of DS to their oncologists, most of the time, oncologists (including myself) do not give specific advice about their use. I have made this observation in different practice settings across the world.

There are a few reasons that may contribute to the lack of action by oncologists against the use of DS by their patients. I will list some of these arguments and my rebuttals.

  1. Argument: “I was taking the supplement before I was diagnosed with cancer, so why stop now?” Counterpoint: If the supplement was not good enough to prevent cancer, what makes us think it will be good to treat the cancer? Furthermore, we oncologists ought to review critically all the medications used and look for any potential interaction with our treatment, or at least eliminate unneeded polypharmacy. There are multiple drugs that may reduce the efficacy of cancer therapeutics that warrant being extremely cautious in reviewing the medication list. The same way we should inquire if the patient needed a daily proton pump inhibitor (which may reduce the efficacy of certain cancer treatments) for an episode of heartburn a few years ago, we need to ask if the patient needs to take a full bag of various DS bottles.  
  2. Argument: “Even if it doesn’t help, it won’t hurt!” Counterpoint: That could not be further from the truth! The inherent side effects and toxicities of DS are well documented in the literature. DS/cancer therapy interaction is a possibility, which may lead to increased toxicity or reduced efficacy. The problem is that even if the physicians checked drug interactions on existing and commonly used applications between DS and cancer therapeutic agents, they would not find any reliable results. Keep in mind that there is no quality control for manufacturing and packaging the DS. Therefore, there is no certainty about the accuracy of the information about the type and dosages of the ingredients in the DS containers. In a recent investigation, 22 out of 25 melatonin gummies were mislabeled about the dose of melatonin with error ranging from having 0 melatonin to an almost 350% higher dose than the label.1 Anyone can go to their backyard and collect whatever plants, leaves, or other items, clean them or not, and put them in a bottle and stick a label on it and sell it as a cancer-curing, immune-boosting supplement. Worse, producers can add to DS active ingredients, poisonous or toxic substances that may lead to health adversities.
  3. Argument: “The treatment I’m receiving for my cancer only has mediocre success/I’ve exhausted most of my treatment options, so I’ll take any advantage I can get.” Counterpoint: There is evidence that some of the DS have anticancer activities. This argument may be the driving reason for many patients to use DS because of their impression of mainstream cancer treatment. They have a point here, as many of the available anticancer treatments have marginal benefits and plenty of side effects, while a DS may have an active ingredient that shows some activity against cancer. To counter this critical and widely held belief, one may argue that these studies are done under laboratory conditions that aren’t replicable in the real world, or were not conducted properly, etc. However, it may be more convincing to accept that DS may have some anticancer effects, and that may be a reason for concern in itself. DS anticancer effect may be limited to slowing the growth of cancer cells, but does not completely kill them, rendering the standard chemotherapy less effective as the cancer cells are not dividing fast, potentially leading to developing a new mechanism of resistance. This is the reason for using hormone therapy sequentially, not concurrently, to adjuvant chemotherapy in breast cancer for the potential risk of antagonist effects of hormone therapy. As always, we must turn to evidence-based medicine to offer the best guidance to our patients. There is emerging evidence that some DS may increase neo-angiogenesis in tumors.2 A real-world experience revealed a worse outcome for patients with breast cancer who took various supplements including antioxidants, iron, and vitamin B12 before or during chemotherapy in a SWOG study of 1,134 patients.3  

I have used different phrases over the years to make my concerns about DS clear to my patients. But I found that the statement that resonates well is this: “If it is good for normal cells, how do we know it is not good for smarter cancer cells?”

As a disclaimer, I am not denying all potential benefits of some DS in some situations. But if we, like DS users, believe that they are active compounds—and many DS are—then we should deal with them as medications. We should take DS seriously during our patients’ consultation and make sure we spend a few extra minutes to give proper advice about them.


  1. Cohen PA, Avula B, Wang YH, et al. Quantity of Melatonin and CBD in Melatonin Gummies Sold in the US. JAMA. 2023;329:1401-2. doi:10.1001/JAMA.2023.2296.
  2. Wang T, Dong Y, Huang Z, et al. Antioxidants stimulate BACH1-dependent tumor angiogenesis. J Clin Invest. Published online August 31, 2023. doi:10.1172/JCI169671.
  3. Ambrosone CB, Zirpoli GR, Hutson AD, et al. Dietary supplement use during chemotherapy and survival outcomes of patients with breast cancer enrolled in a cooperative group clinical trial (SWOG S0221). J Clin Oncol. 2020;38(8):804-814. doi:10.1200/JCO.19.01203.


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Tarek Haykal, MD

Dec, 05 2023 11:33 AM

Thank you for this relevant piece. I do get similar questions in almost every clinic. My response is always I do not know what supplement that is and what it contains, please try your best to avoid them. As medical doctors we can rely on our science and recommend drugs that have undergone rigorous testing for both efficacy and safety

Hao-Wen Sim, FRACP, MBBS

Jan, 01 2024 10:39 PM

Thank you for sharing your insights on dietary supplements and their role in cancer treatment, which resonate with my own views. Open communication between patients and oncologists regarding dietary supplements is crucial. Moving forward, it will be important to focus on vigilant monitoring, collaborations between oncologists and pharmacists, and conducting robust research on promising repurposed medications and supplements.

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