Adverse Effects of Herbal Medicines

Adverse effects of herbal medicines are the unintended and harmful responses that follow the use of plant-based remedies. They include effects caused by a herb's own constituents, effects arising when herbs alter the action of co-administered drugs, and harms that trace to quality problems such as contamination or adulteration. Documenting and classifying these effects is central to challenging the assumption that herbal products are uniformly safe.

Definition

An adverse effect of a herbal medicine is a noxious and unintended response to a plant-based preparation, whether caused by its intrinsic constituents, by interaction with other drugs, or by quality defects in the product.

Scope

The topic covers the main categories of harm associated with herbal medicines: intrinsic (type A and type B) reactions to plant constituents, herb-drug interactions affecting pharmacokinetics or pharmacodynamics, and adverse events attributable to contamination or adulteration. It addresses how such effects are detected — largely through case reports, case series, and pharmacovigilance — and why under-reporting and attribution difficulties make their true frequency hard to estimate. It is a reference-educational overview, not advice on whether to use any product.

Core questions

• What categories of adverse effect are associated with herbal medicines, and how are they distinguished?
• How do herb-drug interactions arise, and which are clinically significant?
• How are adverse effects of herbal medicines detected and reported, and why is their true frequency uncertain?
• How can an adverse event be attributed to a herb's intrinsic toxicity rather than to contamination or adulteration?

Key concepts

• Intrinsic versus extrinsic adverse effects
• Herb-drug interactions
• Pharmacokinetic and pharmacodynamic interactions
• Herbal hepatotoxicity
• Pharmacovigilance and case reporting
• Under-reporting and attribution difficulty
• Causality assessment

Mechanisms

Adverse effects of herbal medicines arise through several mechanisms. Intrinsic effects stem from a plant's own constituents, ranging from predictable, dose-related (type A) reactions to idiosyncratic (type B) ones such as some forms of herbal hepatotoxicity. Herb-drug interactions occur when a herbal constituent changes the pharmacokinetics of a conventional drug — for example by inducing or inhibiting drug-metabolising enzymes or transporters — or its pharmacodynamics, altering its effect at the target; St John's wort is a well-documented example of an enzyme-inducing herb. Other apparent adverse effects actually originate in contamination or adulteration rather than the labelled plant. Because most evidence comes from spontaneous reports, causality assessment and under-reporting are central methodological concerns.

Clinical relevance

Awareness that herbal medicines can cause adverse effects — including liver injury and interactions with prescribed drugs — supports critical appraisal of safety claims and recognition that herbal use is relevant information in any medication history. This topic describes the categories and detection of such effects; it is not a basis for managing an individual patient, which requires clinical judgement.

Epidemiology

The true frequency of adverse effects of herbal medicines is difficult to quantify because much of the evidence comes from case reports and spontaneous pharmacovigilance, which are subject to under-reporting and to uncertainty in attributing an event to a specific herb. Systematic reviews have nonetheless documented serious effects, including psychiatric and neurological reactions and hepatotoxicity, and have catalogued numerous interactions between herbal and prescribed medicines.

History

Systematic documentation of herbal adverse effects expanded markedly from the late twentieth century, as widespread over-the-counter use generated case reports of liver injury, interactions, and other harms. Systematic reviews by Ernst and by Izzo and Ernst consolidated this scattered evidence, and the World Health Organization developed pharmacovigilance guidance to bring herbal safety monitoring into established reporting systems.

Debates

How reliable is case-report evidence for establishing herbal harms?

Most evidence on herbal adverse effects rests on case reports and spontaneous reporting, which support hypothesis generation but make incidence estimation and firm causal attribution difficult.

Key figures

• Edzard Ernst
• Angelo A. Izzo
• Felix Stickel

Seminal works

• ernst-2003-psychiatric
• izzo-ernst-2009
• stickel-2015

Frequently asked questions

Can herbal medicines interact with prescription drugs?

Yes. Herbal constituents can change how a drug is absorbed, metabolised, or eliminated, or alter its effect at the target; systematic reviews have catalogued many such interactions.

Why is the real frequency of herbal adverse effects hard to know?

Most reports are spontaneous case reports, which are under-reported and make it difficult to attribute an event to a specific herb or to estimate how often it occurs.