Researchers have long been trying to find whether or not mistletoe is an effective treatment for cancer. Now, a new study has found proof that it may not be providing any benefit.

Mistletoe is the common name for a group of hemi-parasitic plants in the order Santalales that grow attached to and within the branches of a tree or shrub. In some parts of Europe, its extract is widely used as a therapy for cancer patients.

Mistletoe proponents believe it strengthens the immune system while minimizing the side effects of chemotherapy and radiation therapy, thereby improving survival and increasing quality of life during treatment.

While the plant contains several biologically active substances that could kill cancer cells, fight viruses and tune the immune system, how these substances work is still not clearly understood and there is much debate about whether they work at all in oncology therapy, according to Dr. Markus Horneber, lead review author and member of the Work Group for Biological Cancer Treatment based in Nuremberg, Germany.

In the study, the researchers looked at whether mistletoe extract could increase survival times, enhance the response of tumors to therapy, improve quality of life and alleviate the adverse effects of anti-cancer drugs. The team also evaluated the safety of mistletoe extracts.

The review included 21 randomized clinical trials that used the extract either as the sole therapy or as an adjunct to chemotherapy or radiation.

The studies comprised 3,484 cancer patients from Austria, Bulgaria, China, Germany, Italy, Romania, Russia and Ukraine.

There was little standardization in procedure, patient groups, outcome measures or the mistletoe compounds or doses used, the reviewers found.

In the 13 studies that investigated how long patients survived, six showed some benefit; but again, the authors noted that the methodology of all of these studies was of low quality.

Of the 16 studies that looked at quality of life, reduction of symptoms or reduced side effects from chemotherapy, 14 showed some benefit, but the authors found only two of these studies to have higher-quality methodology.

There was no evidence of side effects or adverse reactions to the mistletoe extract.

While the data supporting mistletoe's benefits are weak, "nevertheless, there is some evidence that mistletoe extracts may offer benefits on measures of [quality of life] during chemotherapy for breast cancer, but these results need replication," the reviewers concluded.

The study is published in The Cochrane Library, a publication of The Cochrane Collaboration, an international organization.