From Memorial Sloan Kettering Cancer Center Herbs and Supplements site (much more in link)

Derived from the seed, pod, or fruit of milk thistle, the flavolignan silymarin is used primarily to manage various liver diseases. In vitro and animal studies suggest that flavonoids in milk thistle have antioxidant and anticancer effects (7) (8) (12) (16) (17), and may protect against Alzheimer's disease (23) (24).

Clinical data suggest benefits of supplementation with milk thistle or silymarin for improving lipid profiles and glycemic indices in type 2 diabetic patients (18) (40). It may also be useful against alcoholic liver disease (9), cirrhosis (11) (19), and for improving fibrosis associated with nonalcoholic steatohepatitis (39). However, in both a large survey and subsequent multicenter placebo-controlled trial among patients with chronic hepatitis C, silymarin did not significantly reduce serum ALT levels (41) (42). In addition, although silymarin was associated with reduced progression from fibrosis to cirrhosis, there was no impact on clinical outcomes was (43). Milk thistle was also reported no more effective than placebo against trichotillomania in a study involving both adults and children (46).

Other studies suggest milk thistle may reduce chemotherapy-induced hepatotoxicity in children with acute lymphoblastic leukemia (20), and cisplatin-induced nephrotoxicity (28) as well as improve capecitabine-induced hand-foot syndrome (6) and radiation therapy-induced mucositis (3). Topical administration of a silymarin gel was reported to reduce the severity of radiodermatitis and delay its occurrence (47).

Both silymarin (27) and silibinin (48) have estrogenic effects with greater affinity for estrogen receptor (ER-beta), and activation of ER-beta results in suppression of cell growth.
Silibinin can exacerbate the negative effects of chronic alcohol consumption on liver cancer (10).

https://www.mskcc.org/cancer-care/integrative-medicine/herbs/milk-thistle

The above says it is estrogenic. Here's more from WebMD:

Does Milk Thistle Have Any Side Effects?

Usually, milk thistle causes few, if any, serious side effects. Studies show that it's safe when taken for up to 41 months.

Milk thistle may cause diarrhea. More rarely, it may cause nausea, bloating, gas, and upset stomach.

Women who are breastfeeding or pregnant should avoid using milk thistle.

If you have a ragweed allergy, you should also avoid milk thistle. Milk thistle may cause a rash or lead to severe allergic reaction.

Since milk thistle may mimic the effects of estrogen, some women should avoid this herb. This includes women who have fibroid tumors or endometriosis. Additionally, women with breast, uterine, and ovarian cancers should not take milk thistle.

Always tell your doctor of any supplement you are taking. Milk thistle may interact with certain medications.

https://www.webmd.com/digestive-disorders/milk-thistle-benefits-and-side-effects#1

The Positive "Estrogenic" Effects of Milk Thistle Extract

Just as the media often creates stories on natural products that lack critical thinking and are fraught with incorrect conclusions, sometimes marketers of dietary supplements make the same mistake. As it relates to milk thistle extract (silymarin), there are warnings out in the marketplace that women avoid milk thistle extract as it increases the risk of breast cancer and has estrogenic effects. That is a scary warning, but is it valid? Not at all, in fact telling women to avoid milk thistle extract is doing them a disservice.

Why the Controversy?

There really is not any controversy. There is simply a misguided warning. Looking at the scientific literature, the overwhelming majority of studies show positive results in breast cancer models as well as those on bone metabolism and zero estrogenic effects have been noted in clinical trials in humans. The warning is based solely upon selected initial in vitro (test tube) and animal models from several years ago. These sorts of early studies have to be interpreted with great caution as they can produce misleading results because of dosage, metabolism, and concentration as well as the strain of rodent used. Follow up studies have completely eliminated any concern. In fact, they show just the opposite—a protective effect.

The most comprehensive and authoritative evaluation on the safety of milk thistle extract was conducted by the National Toxicology Program of the United States Department of Health and Human Services. This study administered milk thistle extracts at varying dosages to different strains of mice and rats for periods up to two years, the human equivalent of lifelong exposure. The dosages used are simply not possible to achieve in humans taking milk thistle extract (e.g., female rates were given a daily dose of 2,750 mg of milk thistle extract per 2.2 pounds body weight). It would be like a woman taking 192,500 mg of milk thistle extract each day instead of the recommended 150 to 300 mg.

So what did the results show? "There was no evidence of carcinogenic activity."

Interestingly, female rats demonstrated decreased breast (mammary) cancer and male rats showed decreased liver cancer with milk thistle extract.

There are other studies that show silymarin is actually quite promising in both the prevention and possible therapeutic aid in breast cancer. Some of the beneficial effects noted include:

• Prevention of the expression of genes and enzymes pivotal in breast cancer development.
• Inhibition of breast cancer cell growth and the inhibition of key pathways that cancer cells use to grow.
• Promotion of apoptosis (programmed cell death) in breast cancer cells.

Milk Thistle's Positive Estrogenic Effects

The effects of plant compounds often referred to as "phytoestrogens" like those in in ground flaxseeds, nuts and seeds, soy, many fruit and medicinal plants on estrogen receptors and metabolism is quite complex. Because there estrogenic effects are so weak, they actually exert more anti-estrogenic effects on breast tissue by occupying estrogen receptors thereby blocking the effects of stronger estrogenic agents including estradiol and pesticides.

On the flip side, many phytoestrogens are able to act on bone metabolism in a very positive manner. Preclinical studies with milk thistle extracts are showing tremendous promise in doing just that. So, milk thistle extracts block the negative effects of estrogen on breast and uterine tissue, while acting in a beneficial way to improve bone health.

Milk thistle components have been shown to exert dual effects on bone health by preventing the breakdown of bone by cells known as osteoclasts while at the same time improving the building of bone by osteoblasts. While these effects have yet to be proven in clinical studies in humans, they do show a positive effect with milk thistle consistent with dietary sources of phytoestrogens.

What Do We Know About the Benefits of Milk Thistle?

Milk thistle extracts (usually standardized to contain 70-80 percent silymarin) have been shown to exert positive effects in promoting improved liver health in double-blind, clinical trials in humans. It has also been shown to be safe and quite helpful in several types of liver disease, including hepatitis and cirrhosis. The therapeutic effect of milk thistle extracts in these disorders has been confirmed by biopsy as well as by clinical and laboratory data. Silymarin protects the liver by:

• Acting as a direct antioxidant and free radical scavenger.
• Increasing the cellular content of antioxidant compounds such as glutathione.
• Stimulating the formation of new liver cells to replaced those that are damaged.

A special extract of milk thistle bound to phosphatidylcholine, silybin phytosome, has been shown to be better absorbed and produce better results than regular milk thistle extract.

The standard dosage for a milk thistle preparation is based on its silymarin content (70–210 milligrams three times daily). For this reason, standardized extracts are preferred. The best results are achieved at higher dosages, that is, 210 milligrams of silymarin three times daily. The typical dosage for silybin phytosome is 80 to 120 mg two to three times daily between meals. The dosage for dried milk thistle seeds is 12 to 15 g daily.

Again, milk thistle extract is extremely well tolerated and is free of significant side effects. It also has no significant interactions with drugs.

There is no existing evidence that milk thistle extract possesses any negative estrogenic effects in humans and the existing preclinical studies overwhelmingly shows it may have tremendous protective effects against breast cancer as well as osteoporosis.

References:

Abenavoli L, Capasso R, Milic N, Capasso F. Milk thistle in liver diseases: past, present, future. Phytother Res. 2010 Oct;24(10):1423-32.

Bosch-Barrera J, Menendez JA. Silibinin and STAT3: A natural way of targeting transcription factors for cancer therapy. Cancer Treat Rev. 2015 Jun;41(6):540-6.

Jacobs BP, Dennehy G, Ramirez J, et al. Milk thistle for the treatment of liver disease: a systematic review and meta-analysis. Am J Med 2002;113:506-15.

Mohd Fozi NF, Mazlan M, Shuid AN, Isa Naina M. Milk thistle: a future potential anti-osteoporotic and fracture healing agent. Curr Drug Targets. 2013 Dec;14(14):1659-66.

National Toxicology Study Group. Toxicology and carcinogenesis studies of milk thistle extract (CAS No. 84604-20-6) in F344/N rats and B6C3F1 mice (Feed Studies). Natl Toxicol Program Tech Rep Ser. 2011 May;(565):1-177

Ramasamy K, Agarwal R. Multitargeted therapy of cancer by silymarin. Cancer Lett. 2008;269: 352-362.

Saller R, Brignoli R, Melzer J, et al. An updated systematic review with meta-analysis for the clinical evidence of silymarin. Forsch Komplementmed 2008;15:9-20.

Great article Marijen. I take Milk Thistle periodically.

I looked it up recently and it interacts with Tamoxifen--my MO pulled me off.

I was taking milk thistle for years before my diagnosis. When I was Put on Tamoxifen Iresearched the contraindications and Milk Thistle is one. It interferes with the effectiveness of tamoxifen and so I'm not taking it anymore. I have one more year on Tamoxifen and then right back on MT.