Milk thistle (Silybum marianum) is a medicinal herb native to Southern Europe. This herbal medicine possesses antioxidant and anti-inflammatory properties and has several health benefits.
Though generally considered safe, it is known to cause adverse effects in some people with long-term use. Individuals using milk thistle have reported abdominal problems, allergic reactions, and interactions with estrogen and certain types of medications.
In this article, we have discussed the possible side effects of milk thistle. If you are taking this herb/supplement, you should be wary of the possible adverse effects it can cause.
Milk Thistle Side Effects
1. May Cause Abdominal Issues
Research states that milk thistle can cause certain abdominal problems like diarrhea, bloating, gas, and an upset stomach. Oral ingestion of milk thistle has also been linked to abdominal fullness (or abdominal pain), anorexia (a loss of appetite), and changes in bowel habits (1).
2. May Cause Allergic Reactions
Milk thistle can cause allergic reactions in people, especially in individuals who are also allergic to ragweed, marigolds, daisies, and chrysanthemums (2).
However, if you develop an allergic reaction after taking milk thistle, stop use and consult your doctor.
3. May Interact With Estrogen
Milk thistle is known to have several estrogen-like properties, and certain sources state that it may aggravate a few health conditions (like endometriosis, where the endometrial tissue appears outside the uterus and causes pain) sensitive to estrogen (4).
Milk thistle might also decrease hormone levels in the body (5). Taking it along with estrogen pills might reduce their effectiveness. Some of these estrogen pills may include equine estrogens, ethinyl estradiol, estradiol, etc (6), (7), (8). However, more direct research is warranted in this regard.
4. May Have Interactions During Breastfeeding And Pregnancy
Though milk thistle has been historically used to improve the flow of breast milk, its benefits during breastfeeding and pregnancy are not well studied yet. Hence, stay safe and avoid use.
5. May Interact With Cholesterol Medication
Milk thistle might interact with statin drugs, which are known to lower cholesterol levels (lipid-lowering) (9). Some of these drugs may include Mevacor, Lescol, Zocor, Pravachol, and Baycol. Milk thistle interacts with these drugs as both are broken down by the same liver enzymes.
6. May Lower Blood Sugar Way Too Much
Milk thistle contains a chemical called silymarin, which can decrease blood sugar levels (10). Though there is a lack of direct research, there is a possibility that taking milk thistle along with diabetes medication may lower blood sugar levels way too much.
7. May Interact With Other Drugs
Certain drugs are broken down in the liver, and milk thistle might decrease how quickly this happens (11). Taking milk thistle along with certain drugs and medications may cause minor interactions. Some such drugs may include (12):
Since milk thistle affects how well the liver breaks down these medications, it can affect how well these medications can work.
Some research also states that overall, milk thistle may not pose a great risk for drug interactions in humans (13).
8. May Cause Weakness
In one case, milk thistle had caused excessive sweating and weakness (14).
Milk thistle, though beneficial, could cause adverse effects in some people. These may include abdominal issues and allergic reactions. While it could also cause drug interactions, research is mixed. If you are on any medication, however, it is safe to consult your doctor before taking milk thistle.
Expert’s Answers For Readers’ Questions
What is the daily upper limit for milk thistle?
Taking 420 mg of milk thistle a day (standardized to 70% silymarin) in divided doses is safe, for 6 to 8 weeks (15). However, consult your doctor for the best advice.
Where to buy milk thistle?
You can buy milk thistle online in the supplement form. You might also get the herb at your nearest supermarket/pharmacy.
How long does it take for milk thistle to start working?
It may take one or two weeks for milk thistle to start working.
Can you drink alcohol while taking milk thistle?
Yes, you can drink alcohol while taking milk thistle. For centuries, this herb has been used in fighting hangovers (9). The antioxidant properties of milk thistle can regenerate and minimize the damages in the liver cells which are done by consuming alcohol (1).
Can you take milk thistle long term?
Yes, you can take milk thistle long term. In animal studies, the antioxidant and radical-scavenging properties of silibinin in milk thistle were found to reduce tumors (16). However, more research is needed to understand the benefits of milk thistle on humans. So, consult a doctor to understand about its long term usage.
- Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects: Summary, AHRQ Evidence Report Summeries, National Center for Biotechnology Information, U.S. National Library of Medicine.
- Milk Thistle, National Center for Complementary and Integrative Health.
- Milk Thistle, StatPearls.
- Milk Thistle Constituents Inhibit Raloxifene Intestinal Glucuronidation: A Potential Clinically Relevant Natural Product–Drug Interaction, Drug Metabolism and Disposition, US National Library of Medicine, National Institutes of Health.
- Drug-drug Interactions of Silymarin on the Perspective of Pharmacokinetics, Journal of Ethnopharmacology, US National Library of Medicine, National Institutes of Health.
- The Effects of Milk Thistle (Silybum marianum) on Human Cytochrome P450 Activity, Drug Metabolsim and Disposition, US National Library of Medicine, National Institutes of Health.
- Review of Clinical Trials Evaluating Safety and Efficacy of Milk Thistle (Silybum marianum [L.] Gaertn.), SAGE Journals.
- Milk thistle: early seeds of potential, The Lancet Oncology, US National Library of Medicine, National Institutes of Health.
- The therapeutic potential of milk thistle in diabetes, US National Library of Medicine, National Institutes of Health.
- Silybum Marianum, Science Direct.
- Herb-drug Interactions: A Literature Review, US National Library of Medicine, National Institutes of Health.
- Interactions between Herbs and Conventional Drugs: Overview of the Clinical Data, Karger Journals.
- Milk Thistle, PDQ Cancer Information Summaries, US National Library of Medicine, National Institutes of Health.
- “Silymarin”, a Promising Pharmacological Agent for Treatment of Diseases, Iranian University of Medical Sciences, US National Library of Medicine, National Institutes of Health.
- Investigating the Potential for Toxicity From Long-Term Use of the Herbal Products, Goldenseal and Milk Thistle, Toxicologic Pathology, US National Library of Medicine, National Institutes of Health.
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