Medical Mistletoe: Can the Holiday Plant Really Fight Cancer?

In some countries, cancer patients take mistletoe injections to ease symptoms, but the exact effects of the extracts are still up for debate

Frost on a sprig of European mistletoe. Carol Sharp/ Ubiquitous/Corbis

During the holidays, mistletoe is famous for helping couples sneak a smooch under a strategically hung sprig. But long before Christ was born, Druids, Greeks and other ancients knew the plant as a powerful healer for ailments from epilepsy to infertility. Today people are again touting mistletoe's benefits as a natural medicine—this time in the fight against cancer.

European mistletoe (Viscum album) is a poisonous and semiparasitic plant that grows on a number of tree species. It is increasingly being processed into extracts that, administered by injection, have become very popular alternative treatments in parts of Europe. But does it actually work? The piles of literature on mistletoe as medicine are so far coming up inconclusive. In the US, mistletoe treatments are currently available only from a few dozen naturopathic clinics, and the extracts are unlikely to win FDA approval anytime soon. The NIH currently recommends against the use of mistletoe as a cancer treatment outside of clinical trials, because it hasn't yet been proven either effective or safe.

Among the dozens of laboratory experiments conducted to date, some credit mistletoe extract with killing cancer cells in animals and boosting the body's immune system, which may help to fight the disease naturally. But other studies have shown little or no benefit. And even when mistletoe has appeared to succeed in the lab, it hasn't been proven through rigorous clinical trials to work reliably in the human body. The U.S. National Cancer Institute's Physician Data Query database cautions that “most clinical studies conducted to date have had one or more major weaknesses that raise doubts about the reliability of the findings.”

Channing Paller, assistant professor of oncology at the Johns Hopkins University School of Medicine, hopes to help crack the medical mystery. She's the principal investigator for a planned Hopkins study that will analyze mistletoe extract's safety and dosage recommendations in people. Paller is especially interested in examining mistletoe's claimed effectiveness as a form of immunotherapy, meaning something that boosts the body's immune system to help it fight disease. (The trial is largely funded by public donations to Believe Big, a nonprofit founded by cancer survivor Ivelisse Page. Page credits mistletoe treatment with helping her beat Stage 4 colon cancer in 2008.)

“We're interested in testing whether mistletoe in humans has an effect on the immune system,” she says. “And if so, can we find a biomarker in the blood to see exactly how it might be helping patients?” Later phases of the Hopkins trial will examine mistletoe's efficacy in battling cancer, but the process will take years.

In Germany and other European nations, it's a far different story. The German agency responsible for regulation of herbs has approved mistletoe treatments—not as a cancer fighter but as a palliative treatment that eases symptoms and improves quality of life. During the past few decades many trials have reported that mistletoe helped chemotherapy patients by easing fatigue, nausea and depression while boosting concentration and emotional well-being.

Paller has seen some anecdotal evidence of that in her own practice. “I have a few patients who get mistletoe from other practitioners and use it and they just feel better, they feel like they have more energy overall,” she says. Still, others gave up the treatment because they didn't feel it helped them.

Some European studies also suggest that mistletoe can diminish the toxicity from cancer treatments. That means that when using the extracts, patients are able to tolerate higher doses of chemotherapy, which in turn are able to kill off more cancer. One 2013 study showed hospitalization rates for aggressive chemotherapy declined from 54 percent to 24 percent among people who were using a mistletoe extract known as iscador. 

The pairing of “natural” medicines and more conventional chemotherapy isn't as odd as it might sound at first blush. Paller has worked on Hopkins studies of other naturally derived treatments, including the use of pomegranate and extract of muscadine grape skins for prostate cancer treatment. To her, a famous holiday plant isn't surprising as a potential source of helpful medication. 

“Nature is great at coming up with these compounds. Widely used chemotherapy drugs, like Taxol, came from trees,” such as the Pacific yew tree, she says. But Paller cautions people against getting their hopes up about natural remedies unless they have been through rigorous testing. “There is a lot of quackery out there in the world of natural medicines," she says. "Most compounds that kill cancer cells in the laboratory do not work as well in humans. And they may be harmful, either alone or through interactions with other drugs or foods. We want to make sure they are safe and that they work before recommending them to our patients, so we are applying the same kind of rigorous testing that we would do during any other drug development, whether it comes from a plant or a lab.”

With the wait for further testing, of course, comes a warning not to take matters into your own hands when it comes to mistletoe treatment options. Kissing under the mistletoe may be an exhilarating experience, but eating or otherwise consuming it won't be—the plant is toxic and can cause vomiting or far more serious health effects.

Get the latest Science stories in your inbox.