11 Things to Know About Supplements and Cannabis for Breast Cancer

Between 50% and 85% of breast cancer survivors use supplements after their diagnosis, but there’s still a lot to learn regarding their safety and effectiveness – in part because supplements are regulated by the U.S. Food and Drug Administration not as drugs but as food. 

Increasing numbers of cancer survivors — up to 30% — are also using cannabis for pain, insomnia, stress, or nausea. But research is limited on that, too: there’s little data on recommended formulation, dose, and duration of use. 

These are the challenges laid out by Heather Greenlee, ND, PhD, MPH, a professor in the Public Health Sciences and Clinical Research Divisions at the Fred Hutchinson Cancer Center and an associate professor of medicine in the Division of Hematology and Oncology at the University of Washington School of Medicine.

Here are 11 things we learned from her talk at the 2024 San Antonio Breast Cancer Symposium.

Vitamin D is low-risk, doesn’t interact with many cancer drugs, and has several benefits, including boosting bone health, immune function, and mood. Some studies show better breast cancer outcomes in women who have normal levels of vitamin D in their blood when they are diagnosed (though they don't prove that taking vitamin D supplements will lower your risk of recurrence). You'll want to have your Vitamin D levels tested, though, before and while taking it.

Multivitamins may affect how the body absorbs certain medicines for people receiving treatment for breast cancer. According to one observational study — which is a study where researchers collect data by observing participants without intervening or changing any variables — iron and B12 supplements led to worse outcomes for people with breast cancer.

Many supplements that may seem harmless could actually change how cancer treatments work. Some observational studies have found that turmeric and melatonin interact with CYP enzymes, which help the body metabolize tamoxifen and other medicines. 

Some mushrooms that are marketed as immune boosters — like reishi, turkey tail, and lion’s mane — may cause liver damage and bleeding. 

Other supplements that have been found to be ineffective include aloe vera and hyaluronic acid (used for radiation skin reactions), glutamine (used for nausea and vomiting during chemo), guarana and acetyl-l-carnitine (used for fatigue), and soy (used for hot flashes). Acetyl-l-carnitine, when used for chemo-induced neuropathy, also carries “a moderate or high certainty that the harms outweigh the benefits.” 

These two plants are recommended based on an evaluation of the research. During treatment, lavender aromatherapy has been shown to help ease anxiety, while 2000 mg daily of American ginseng (Panax quinquefolius) may reduce symptoms of cancer-related fatigue. That said, the evidence isn’t that strong — there was much stronger evidence supporting other interventions, like mindfulness (for anxiety) and exercise (for fatigue).

Doctors and researchers have concerns about people in treatment for breast cancer using cannabis when there’s so little research on its safety. Studies have been mostly theoretical, or in cells, or animals — not people. Big questions remain about both short- and long-term impacts of cannabis use during and after active treatment.  

The “most compelling” evidence supports the use of cannabis for chemotherapy-induced nausea and vomiting. However, the evidence is not considered that strong.

Similarly to some supplements, cannabis may interact with CYP enzymes, which help metabolize many breast cancer drugs, including tamoxifen, Verzenio (abemaciclib), Cytoxan (cyclophosphamide), everolimus (Afinitor), Aromasin (exemestane), Femara (letrozole), and others). It may also interact with a protein called P-glycoprotein, important for treatment with tamoxifen and everolimus (Afinitor).

Other holistic options to manage treatment side effects have proven benefits and virtually no risks. Exercise is effective at reducing fatigue during and after treatment, as is cognitive behavioral therapy (CBT). There’s strong evidence showing the benefits of mindfulness-based interventions during and after treatment for both depression and anxiety. And acupuncture has been shown to help with pain. 

Should you avoid supplements and cannabis, then? Not necessarily. Greenlee emphasized the importance of shared decision-making with your providers and caregivers so that you can discuss the current evidence on risks and benefits, and make a decision that’s right for you.