Dietary Supplements and Herbal Products During Anticancer Treatment: Prevalence, Interactions and the Problem of Non-Disclosure. A Narrative Review

Objectives: Dietary supplements and herbal products are widely used by patients undergoing anticancer treatment, and a considerable proportion of this use is never mentioned to their physician. This review summarises current evidence on the prevalence of such use, the pharmacological basis of interactions with anticancer agents, and the factors underlying non-disclosure on the part of both patients and clinicians. Methods: PubMed/MEDLINE and Google Scholar were searched for publications from January 2011, and July 2026, using combinations of terms relating to dietary supplements, herbal medicine, oncological treatment, herb–drug interactions and physician–patient communication. Priority was given to systematic reviews, meta-analyses and studies conducted in populations receiving anticancer treatment. Key findings: Reported prevalence varies widely between studies, depending on which products are counted and on the populations examined. Interactions arise principally through effects on drug metabolism and transport. Patients most often cite the absence of any question from the clinician as their reason for not disclosing; clinicians, in turn, describe their own knowledge of these products as insufficient. Training programmes have been developed to address this, but their effectiveness has yet to be established. Conclusions: Interactions between anticancer agents and dietary supplements or herbal products may be clinically relevant, yet often go unrecognised because their use is not disclosed. Non-disclosure is better understood as a feature of the consultation than as a behaviour of the patient. Two forms of intervention follow: strengthening clinicians' competence, and making enquiry about supplement use routine and free of judgement.