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Milk Thistle Side Effects and Warnings

[Edit] [Revisions] [Writers] Side Effects and Warnings

Safety Issues

Milk thistle is believed to possess very little toxicity. Animal studies have not shown any negative effects even when high doses were administered over a long period of time. 1 A study of 2,637 participants reported in 1992 showed a low incidence of side effects, limited mainly to mild gastrointestinal disturbance. 2 However, on rare occasions severe abdominal discomfort may occur. 3 On the basis of its extensive use as a food, milk thistle is believed to be safe for pregnant or nursing women and researchers have enrolled pregnant women in studies. 4 However, safety in young children, pregnant or nursing women, and individuals with severe renal disease has not been formally established.

No drug interactions are known. However, one report has noted that silibinin can inhibit a bacterial enzyme called beta-glucuronidase, which plays a role in the activity of certain drugs, such as oral contraceptives. 5 This could theoretically reduce their effectiveness.

Interactions You Should Know About

If you are taking:

  • Medications that could damage the liver, such as acetaminophen, phenytoin (Dilantin), alcohol, and phenothiazines: Milk thistle might be protective for some of these drugs.
  • Oral contraceptives : Milk thistle might reduce their effectiveness.


  1. Awang D. Milk thistle. Can Pharm J. 1993;126:403-404.
  2. Albrecht M, Frerick H, Kuhn U, et al. Therapy of toxic liver pathologies with Legalon [in German]. Z Klin Med. 1992;47:87-92.
  3. Adverse Drug Reactions Advisory Committee. An adverse reaction to the herbal medication milk thistle ( Silybum marianum ). Med J Aust. 1999;170:218-219.
  4. Giannola C, Buogo F, Forestiere G, et al. A two-center study on the effects of silymarin in pregnant women and adult patients with so-called minor hepatic insufficiency [in Italian]. Clin Ther. 1985;114:129-135.
  5. Kim DH, Jin YH, Park JB, Kobashi K. Silymarin and its components are inhibitors of beta-glucuronidase. Biol Pharm Bull. 17(3):443-5.