Mistletoe preparations have been used in oncology for decades. Available evidence indicates potential benefits primarily in patient-reported outcomes, such as quality of life (QOL), while convincing effects on hard clinical endpoints, particularly overall survival (OS), remain unproven.

In clinical practice, mistletoe is not the standard antitumor therapy. It may be considered in selected cases as a complementary approach, provided clinicians clearly communicate the expected benefits, limitations, and potential interactions and adverse effects.

A key limitation is the heterogeneity of the available formulations. Extracts derived from Viscum album are administered in different preparations, doses, and routes, most commonly subcutaneously. This variability complicates cross-study comparisons and limits their ability to draw definitive conclusions.

Biologic Basis

From a biologic perspective, potential mechanisms are plausible. Preclinical studies have shown that mistletoe extracts and components, such as lectins, viscotoxins, and polyphenols, show immunomodulatory and antiproliferative effects.

Cellular and animal models have demonstrated the induction of apoptosis, modulation of cytokine production, and activation of innate and adaptive immune responses. These findings suggest increased immune surveillance of tumor cells. These studies are scientifically relevant and may serve as a foundation for further research. However, the significance of preclinical data should not be overlooked. The transition from biological activity observed in the laboratory to measurable clinical benefits in patients remains significant.

Clinical Evidence

Clinical data indicate modest benefits for patient-centered outcomes. A 2020 systematic review and meta-analysis reported a moderate improvement in QOL among patients with cancer. Improvements were also observed in pain, nausea, and overall well-being.

Interpretation remains limited because of the heterogeneity and risk for bias. Variability in the study design, methodological quality, and interventions weakens the strength of the evidence. A statistical signal indicating symptom improvement does not establish robust clinical efficacy.

Evidence for the benefit of OS is less consistent. A 2024 analysis found that clear survival advantages were driven largely by studies with low methodological quality. No significant improvement in OS was observed in the high-quality randomized trials.

The randomized, double-blind, placebo-controlled MISTRAL trial in advanced pancreatic cancer showed no clinically meaningful improvement in OS or global health-related QOL with mistletoe therapy compared with placebo.

Observational analyses, including real-world studies in non-small cell lung cancer treated with PD-1 or PD-L1 inhibitors, suggested longer survival with adjunct mistletoe therapy than with no adjunct therapy. These findings require confirmation in well-designed randomized trials because nonrandomized data are prone to bias.

Guideline View

The S3 guideline“Complementary Medicine in the Treatment of Oncology Patients” fundamentally distinguishes between two possible treatment objectives: a tumor-inhibiting or life-prolonging effect and an improvement in QOL.

Regarding OS, the researchers remained cautious. Given the heterogeneity of the data, they found no sufficient basis for a clear assessment. However, they noted that subcutaneous administration of a total mistletoe extract could be considered to improve the QOL.

Safety Profile

However, data on drug interactions remain limited. Available evidence offers partial reassurance. Researchers have found no evidence of a clinically relevant risk for cytochrome P450 (CYP) mediated pharmacokinetic interactions. In vitro studies using standardized mistletoe products showed no meaningful inhibition of key CYP isoenzymes and no reduction in the efficacy of commonly used cytostatic drugs.

Combination with immune checkpoint inhibitors calls for careful consideration. Given the immunologic activity of mistletoe extracts, a theoretical effect on both beneficial and adverse immune responses is possible. A small observational study published in 2017 reported no increase in adverse events with combined use. However, the authors noted that immunologic interactions could not be excluded.

Side Effects

The safety profile of mistletoe therapy is manageable. According to the National Cancer Institute (NCI) in the US, mistletoe extract (PDQ) and local injection site reactions are among the most common adverse effects. Flu-like symptoms, including fever, headache, and chills, have also been reported. Fatigue, mild gastrointestinal discomfort, and their effects on QOL have also been described.

Allergic reactions, including anaphylactic shock, are rare but clinically relevant. NCI also describes isolated cases of thrombophlebitis and lymph node swelling. Reversible hepatotoxicity has been observed with high doses of recombinant mistletoe lectin.

Intravenous administration requires caution. A phase 1 study of previously treated individuals with solid tumors reported treatment-related adverse events in approximately 62% of participants. The most common adverse events included fatigue, nausea, and chills. Grade 3 or higher treatment-related events occurred in approximately 15% of patients.

Clinical Relevance

Overall, any potential benefit of mistletoe therapy appears to lie in supportive care rather than direct tumor control. Discussions with patients should focus on possible improvements in fatigue, general well-being, nausea, pain, and QOL.

This story was translated from Medscape’s German edition.