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Evidence Report/Technology Assessment: Number 21

Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects


Under its Evidence-based Practice Program, the Agency for Healthcare Research and Quality (AHRQ) is developing scientific information for other agencies and organizations on which to base clinical guidelines, performance measures, and other quality improvement tools. Contractor institutions review all relevant scientific literature on assigned clinical care topics and produce evidence reports and technology assessments, conduct research on methodologies and the effectiveness of their implementation, and participate in technical assistance activities.

Overview / Reporting the Evidence / Methodology / Findings / Future Research / Availability of Full Report


This evidence report details a systematic review summarizing clinical studies of milk thistle in humans. The scientific name for milk thistle is Silybum marianum. It is a member of the aster or daisy family and has been used by ancient physicians and herbalists to treat a range of liver and gallbladder diseases and to protect the liver against a variety of poisons.

Two areas are addressed in the report:

  1. Effects of milk thistle on liver disease of alcohol, viral, toxin, cholestatic, and primary malignancy etiologies.
  2. Clinical adverse effects associated with milk thistle ingestion or contact.

The report was requested by the National Center for Complementary and Alternative Medicine, a component of the National Institutes of Health, and sponsored by the Agency for Healthcare Research and Quality.

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Reporting the Evidence

Specifically, the report addresses 10 questions regarding whether milk thistle supplements (when compared with no supplement, placebo, other oral supplements, or drugs):

One question addresses the constituents of commonly available milk thistle preparations, and three questions address the common and uncommon symptomatic adverse effects of milk thistle.

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Search Strategy

Eleven electronic databases, including AMED, CISCOM, the Cochrane Library (including DARE and the Cochrane Controlled Trials Registry), EMBASE, MEDLINE, and NAPRALERT, were searched through July 1999 using the following terms:

An update search limited to PubMed was conducted in December 1999. English and non-English citations were identified from these electronic databases, references in pertinent articles and reviews, drug manufacturers, and technical experts.

Selection Criteria

Preliminary selection criteria regarding efficacy were reports on liver disease and clinical and physiologic outcomes from randomized controlled trials (RCTs) in humans comparing milk thistle with placebo, no milk thistle, or another active agent. Several of these randomized trials had dissimilar numbers of subjects in study arms, raising the question that these were not actually RCTs but cohort studies. In addition, among studies using nonplacebo controls, the type of control varied widely. Therefore, qualitative and quantitative syntheses of data on effectiveness were limited to placebo-controlled studies. For adverse effects, all types of studies in humans were used to assess adverse clinical effects.

Data Collection and Analysis

Abstractors (physicians, methodologists, pharmacists, and a nurse) independently abstracted data from trials; a nurse and physician abstracted data about adverse effects. Data were synthesized descriptively, emphasizing methodologic characteristics of the studies, such as populations enrolled, definitions of selection and outcome criteria, sample sizes, adequacy of randomization process, interventions and comparisons, cointerventions, biases in outcome assessment, and study designs. Evidence tables and graphic summaries, such as funnel plots, Galbraith plots, and forest plots, were used to examine relationships between clinical outcomes, participant characteristics, and methodologic characteristics. Trial outcomes were examined quantitatively in exploratory meta-analyses that used standardized mean differences between mean change scores as the effect size measure.

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Mechanisms of Action

Evidence exists that milk thistle may be hepatoprotective through a number of mechanisms: antioxidant activity, toxin blockade at the membrane level, enhanced protein synthesis, antifibriotic activity, and possible anti-inflammatory or immunomodulating effects.

Preparations of Milk Thistle

The largest producer of milk thistle is Madaus (Germany), which makes an extract of concentrated silymarin. However, numerous other extracts exist, and more information is needed on comparability of formulations, standardization, and bioavailability for studies of mechanisms of action and clinical trials.

Benefit of Milk Thistle for Liver Disease

Adverse Effects

Adverse effects associated with oral ingestion of milk thistle include:

However, causality is rarely addressed in available reports. For randomized trials reporting adverse effects, incidence was approximately equal in milk thistle and control groups.


Clinical efficacy of milk thistle is not clearly established. Interpretation of the evidence is hampered by poor study methods and/or poor quality of reporting in publications. Problems in study design include heterogeneity in etiology and extent of liver disease, small sample sizes, and variation in formulation, dosing, and duration of milk thistle therapy. Possible benefit has been shown most frequently, but not consistently, for improvement in aminotransferases and liver function tests are overwhelmingly the most common outcome measure studied. Survival and other clinical outcome measures have been studied least often, with both positive and negative findings. Available evidence is not sufficient to suggest whether milk thistle may be more effective for some liver diseases than others or if effectiveness might be related to duration of therapy or chronicity and severity of liver disease. Regarding adverse effects, little evidence is available regarding causality, but available evidence does suggest that milk thistle is associated with few, and generally minor, adverse effects.

Despite substantial in vitro and animal research, the mechanism of action of milk thistle is not fully defined and may be multifactorial. A systematic review of this evidence to clarify what is known and identify gaps in knowledge would be important to guide design of future studies of the mechanisms of milk thistle and clinical trials.

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Future Research

The type, frequency, and severity of adverse effects related to milk thistle preparations should be quantified. Whether adverse effects are specific to dose, particular preparations, or additional herbal ingredients needs elucidation, especially in light of equivalent frequencies of adverse effects in available randomized trials. When adverse effects are reported, concomitant use of other medications and product content analysis should also be reported so that other drugs, excipients, or contaminants may be scrutinized as potential causal factors.

Characteristics of future studies in humans should include:

There also should be detailed attention to preparation, standardization, and bioavailability of different formulations of milk thistle (e.g., standardized silymarin extract and silybin-phosphatidylcholine complex).

Precise mechanisms of action specific to different etiologies and stages of liver disease need explication. Further mechanistic investigations are needed and should be considered before, or in concert with, studies of clinical effectiveness. More information is needed about effectiveness of milk thistle for severe acute ingestion of hepatotoxins, such as occupational exposures, acetaminophen overdose, and amanita poisoning.

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Availability of Full Report

The full evidence report from which this summary was derived was prepared by the San Antonio Evidence-based Practice Center based at The University of Texas Health Science Center at San Antonio and the Veterans Evidence-based Research, Dissemination, and Implementation Center (VERDICT), a Veterans Affairs Health Services Research and Development Center of Excellence under contract No. 290-97-0012. Printed copies may be obtained free of charge from the AHRQ Publications Clearinghouse by calling 800-358-9295. Requesters should ask for Evidence Report/Technology Assessment Number 21, Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects (AHRQ Publication No. 01-E025).

The Evidence Report is also online on the National Library of Medicine Bookshelf, or can be downloaded as a zipped file.

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AHRQ Publication Number 01-E024
Current as of September 2000

Internet Citation:

Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects. Summary, Evidence Report/Technology Assessment: Number 21, September 2000. Agency for Healthcare Research and Quality, Rockville, MD. http://www.ahrq.gov/clinic/epcsums/milktsum.htm

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