When you think of mistletoe, you might picture a holiday season kiss beneath a festive decoration. But the red-berried plant we’re so familiar with has a cousin that’s known for its possible therapeutic value. European white berry mistletoe has been studied for its potential to improve quality of life for people who have cancer.

European mistletoe grows in in the U.K., continental Europe, and Western Asia. For more than 2,000 years, its twigs and leaves have been used in herbal remedies. Celtic druids considered the plant to be a treatment for many illnesses. In the early 20th century, Rudolf Steiner, a practitioner of alternative medicine, and Dr. Ita Wegman began to use mistletoe extract to treat cancer.

Today, mistletoe is among the most widely studied alternative therapies for cancer, according to the National Cancer Institute (NCI). Even so, the medical community remains uncertain about whether it’s an effective treatment. The types of cancers treated vary.

There have been many studies on the use of mistletoe to treat cancer, including clinical trials with people. Most clinical trials have suggested that mistletoe can be an effective treatment for some types of cancer. But the NCI notes that these studies have major weaknesses. That means the findings may not be accurate.

In a 2009 review, researchers found that mistletoe extracts could boost survival rates among people with cancer. Other studies suggest that mistletoe may reduce tumor growth and support the immune system. But all of these studies had limits that make the findings unreliable. The authors of the 2009 review suggest that high-quality studies are needed to learn more about mistletoe’s benefits.

Mistletoe may have benefits for people with certain types of cancer. For people with breast cancer, mistletoe may help improve their physical and emotional well-being. One study showed that mistletoe reduced side effects of chemotherapy, such as nausea, numbness, and the feeling of pins and needles. Some study participants even reported less hair loss. They also felt less worried and depressed, and more hopeful.

Mistletoe may also help people feel less tired when they’re going through radiation therapy. It may help them sleep better as well. A study involving 220 patients with breast, ovarian, and lung cancer showed that those given mistletoe experienced less fatigue, insomnia, anorexia, and nausea. For stomach cancer, adding mistletoe to an oral chemotherapy regimen may also be beneficial. In one study, mistletoe lowered the frequency of diarrhea compared to those who weren’t given the extract.

Mistletoe is available in Switzerland, the Netherlands, and the U.K. It’s most often sold under the drug names Iscador and Helixor. In Germany, mistletoe injections are approved as a treatment to lessen symptoms of tumors and improve the way patients feel.

In the United States, the Food and Drug Administration (FDA) has not approved mistletoe injections to treat cancer. That means they aren’t available to the public. Mistletoe extracts can be used to treat people who are involved in clinical trials.

Diluted forms of mistletoe extract are available in the United States. These products contain small amounts of the plant, mixed with water or alcohol. If you want to try taking mistletoe extracts, speak to your doctor. Mistletoe has a wide range of possible side effects. You should only take it under a doctor’s care.