What Are The Side Effects Of Milk Thistle?

Reviewed by Merlin Annie Raj, Registered Dietitian
by Ravi Teja Tadimalla

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).

Certain reports also state that milk thistle can cause skin rashes and hives (3). How this happens is yet to be studied.

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):

  • Indinavir
  • Elavil
  • Valium
  • Celebrex
  • Voltaren
  • Lescol
  • Cozaar
  • Demadex
  • Coumadin
  • Zyflo

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

Certain individuals might experience weakness with milk thistle. Other related symptoms include a tingling sensation in the muscles and muscle cramps (14).

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.

16 sources

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Ravi Teja Tadimalla

Ravi Teja Tadimalla is a Senior Content Writer who specializes in writing on Health and Wellness. He graduated from SRM University, Chennai, and has been in the field for well over 4 years now. His work involves extensive research on how one can maintain better health through natural foods and organic supplements. Ravi has written over 250 articles and is also a published author. Reading and theater are his other interests.