Hidden Dangers: Pharmacists at the Forefront of Managing Drug-Herb Interactions in Cancer Patients

A significant and often-overlooked threat to the safety and efficacy of cancer treatment lies in the common practice of using herbal supplements. A new narrative review from the University of California (UC) Davis highlights the critical role of oncology pharmacists in identifying and mitigating these drug-herb interactions. The review, based on a survey of their own patient population, provides detailed, evidence-based recommendations on five of the most frequently used supplements: curcumin, reishi mushroom, quercetin, elderberry, and maitake mushroom.

The Pervasive and Perilous Practice of Supplement Use

An alarming number of cancer patients—with some estimates as high as 78%—turn to herbal supplements. They do so for a variety of reasons, from managing the side effects of chemotherapy to seeking alternative or complementary therapies. Compounding the risk is the fact that up to 70% of patients do not disclose their supplement use to their oncology providers, often believing that “natural” products are inherently safe.

This issue is especially critical in the modern era of cancer care, which has seen a rapid expansion of targeted oral therapies. While these treatments offer patients convenience and flexibility, they also present a heightened risk of drug-herb interactions. Many of these interactions occur because supplements can interfere with key metabolic pathways, such as the cytochrome P450 (CYP450) enzyme system, which is responsible for breaking down a large number of oral cancer medications.

The UC Davis Study: Uncovering the Problem and Its Solution

To understand this issue better, pharmacists at UC Davis conducted a survey-based review of 100 patients receiving oral cancer therapy who were also using herbal supplements. The study’s findings underscored the vital role of the pharmacist. The most common method of discovering supplement use was through pharmacist-led medication reconciliation, which accounted for 57% of cases. The pharmacists’ recommendations to either continue or discontinue a supplement were universally accepted by providers (100% acceptance) and met with high acceptance rates from patients (94%).

The study identified over 40 different herbal supplements in use, with the top five being the focus of their detailed review.

A Closer Look at the Five Most Common Supplements

The review provides a detailed breakdown of the risks associated with the five most frequently encountered supplements:

Curcumin: Extracted from turmeric, curcumin is studied for its potential anti-cancer and anti-inflammatory properties. However, the review highlights significant concerns, including its unpredictable bioavailability, ability to inhibit CYP450 enzymes, and its antioxidant effects. The latter is particularly problematic as some chemotherapies rely on oxidative stress to kill cancer cells. The authors recommend against the regular use of curcumin in oncology patients until further studies can confirm its safety and benefit.
Reishi Mushroom: Used for centuries in traditional medicine, the reishi mushroom is prized for its proposed antioxidant and antitumor effects. While it generally lacks toxicity, the review notes potential interactions with anticoagulants and antiplatelet agents. Its theoretical antioxidant effects also raise concerns about interfering with certain chemotherapies. The recommendation is to use it with caution and after careful consideration of a patient’s other medications.
Quercetin: A flavonoid found in many common fruits and vegetables, quercetin is marketed for its purported health benefits. However, it is a significant inhibitor of several CYP450 enzymes and drug elimination pathways. For patients on oral EGFR inhibitors like osimertinib, concurrent quercetin use could lead to dangerously high drug levels and severe toxicity. The review strongly advises caution and a thorough evaluation of potential interactions.
Elderberry: Commonly used for cold and flu symptoms, elderberry is generally well-tolerated. The review found limited data on drug-herb interactions, though it cites a case report suggesting a possible interaction with the oral cancer drug pazopanib. Given its potential as an immunostimulant, the authors advise caution for patients undergoing immunotherapy, as it could theoretically increase the risk of immunotherapy-related side effects.
Maitake Mushroom: This edible fungus is studied for its immunomodulatory and antitumor properties. While it appears to be well-tolerated, a case report noted that it increased the INR (a measure of blood clotting time) in a patient on warfarin, indicating a risk for other drugs with similar protein-binding properties. As it affects the immune system, the review recommends against its use in combination with immunotherapy.

The Way Forward: Empowering Pharmacists in Oncology

The review concludes that pharmacists are indispensable in this complex area of care. They are encouraged to use established, evidence-based resources from organizations like the National Comprehensive Cancer Network and the Memorial Sloan Kettering Cancer Center’s herbs database to guide their recommendations. Furthermore, they should advise patients to be wary of proprietary supplement blends, which often lack transparency, and to use specific, non-judgmental language when discussing supplements to foster open communication.

By actively engaging with patients and providers, pharmacists can ensure that all medications and supplements are carefully vetted, ultimately preventing dangerous interactions and protecting patients throughout their cancer journey.