Abstract
Natural product (NP) use by patients alongside their conventional cancer therapies is ubiquitous. This common, yet often hidden, practice can potentially contribute to significant patient harm, given the narrow therapeutic window of most anticancer drugs. This review takes on this challenge directly, moving past theoretical concerns to summarize current clinical evidence on interactions between widely used NPs and modern cancer treatments, including chemotherapy, targeted therapy, and immunotherapy. We break down the key pharmacokinetic (PK) mechanisms, such as the disruption of cytochrome P450 enzymes, and the pharmacodynamic (PD) effects that can either help or hinder treatment. By examining both well-established clinical interactions and those supported by preliminary or preclinical findings, we highlight how NPs may alter the effectiveness of anticancer medications and where evidence remains uncertain. Lack of reliable safety information for NPs along with widespread use of these products by patient populations has the potential to impact clinical care and patient outcomes significantly, frequently causing harm. We advocate for improved patient-provider communication and additional evidence-based research to address this gap in literature. The majority of reported interactions are based on preclinical or limited clinical evidence. A more rigorous evidence base including real-world data and clinical trials is urgently needed to guide practice.