Supplement use with cancer treatment: Helpful or harmful?

Hoping to improve their symptoms and outcomes, many patients with cancer consume antioxidants, herbs, and other dietary supplements while undergoing radiotherapy and chemotherapy—usually without telling the clinicians who are caring for them. Oncology team members should seek open discussions with patients about these alternative treatments, including the unknowns and potential risks associated with their use during cancer treatment.

Although herbal and other dietary supplements are little regulated, these substances can have important, and in some cases potentially dangerous, metabolic and drug interactions.^1-4 Reports of promising preclinical findings indicating that mixtures of soy isoflavones may mitigate radiotherapy toxicities in the lung are likely to spark increased patient interest in complementary and alternative medicines (CAMs) such as dietary supplements and herbal remedies in general. But not all supplements are as promising, and some may even diminish the therapeutic benefits of conventional cancer treatments.

USE OF CAM PRODUCTS IS COMMON

Recent studies indicate that more than a third—possibly more than half—of adult patients with cancer consume complementary medicine supplements while undergoing radiotherapy and chemotherapy, including herbal teas, vitamins and antioxidants, and herbal supplements such as traditional Chinese plant medicines.^5,6 Usage is even higher among specific patient populations such as women with breast cancer; usage is prevalent even in children, as up to 84% of pediatric cancer patients are given complementary medicines.⁷

These substances can be expensive, with older adult patients spending as much as $300 a month on CAM products.^6,8 Surveys also reveal that many patients who consume these products do so every day, in the expectation that the products will increase the efficacy of chemotherapy or radiotherapy and extend survival time.^5,6 Such optimism about outcomes is out of step with a scant research evidence base, and appears to involve the assumption by patients and advocates that supplements are, at worst, ineffective.

Even more significant, most patients surveyed do not discuss their use of these substances with clinicians.^5,6 One recent survey of patients undergoing radiotherapy found that 44% used CAM products while undergoing treatment, but only 12% discussed this with their radiation oncology clinicians.⁹ Female patients and those with full-time jobs are less likely to discuss the issue with clinicians than are other patients.⁹

MORE HARMFUL THAN HARMLESS

This is a potentially important gap in clinical communication, treatment planning, and patient education. Some of these substances can affect drug metabolism and, potentially, the efficacy of conventional anticancer treatments. For example, one study revealed that 29% of patients undergoing radiotherapy consume supplementary antioxidants, which can diminish the free-radical tumor-killing effects of irradiation.^6,10

Clinical data suggests that antioxidants such as β-carotene and α-tocopherol—a form of the antioxidant vitamin E—can reduce radiotherapy-associated toxicity, but at the life-threatening cost of reduced tumor control.¹⁰ Antioxidant supplements are associated with increased disease recurrence and cancer-specific mortality, as well as reduced overall survival, among smokers undergoing radiotherapy; however, these effects are not associated with nonsmokers undergoing radiotherapy.¹⁰

The available scientific literature regarding the effects of dietary supplements during cancer treatment is contradictory and overwhelmingly preclinical.¹¹ (A prohibition on federally funded medical marijuana research has prevented robust development of an evidence base on marijuana use among cancer patients.¹²) Clearance and circulating concentrations of medications that are metabolized by P450 enzymes (eg, CYP3A4) might be modulated by herbs that inhibit P450 activity, such as goldenseal, Ginkgo biloba, grape seed, milk thistle, and ginseng extracts.¹³ Their use has potentially important but poorly understood implications for chemotherapy and radiochemotherapy. Furthermore, the constituent and active ingredients of popular herbal supplements, such as black cohosh, grape seed and green tea extracts, and gingseng, vary between commercial brands, resulting in inconsistent and variable dosing and making it difficult to extrapolate research outcomes to patients’ dosing and outcomes.¹⁴ For example, green tea extract appears to interact significantly with P450 CYP3A4-metabolized drugs, but to varying degrees depending on the brand of extracts involved.¹⁴

Preclinical research also suggests that vitamin C (ascorbic acid) may reduce the bioavailability of bortezomib (Velcade) and that black cohosh increases tamoxifen toxicity, while immunomodulating supplements (eg, echinacea) might diminish the effects of cancer immunotherapies.¹⁵