No. The estrogenic effects of silymarin are well-studied. Below is the abstract of a study that found no binding of silymarin to the estrogen receptor alpha (ERalpha). Binding to ERalpha would be a cause for concern in estrogen-sensitive breast or uterine cancer patients. Silymarin is an agonist (positive effector) for the ERbeta receptor, whereby it can positively affect bone density.
Reference: Seidlova-Wuttke, D.; Becker, T.; Christoffel, V.; Jarry, H.; Wuttke, W. Silymarin is a selective estrogen receptor beta (ERbeta) agonist and has estrogenic effects in the metaphysis of the femur but no or antiestrogenic effects in the uterus of ovariectomized (ovx) rats. J. Steroid Biochem. Mol. Biol. 86:179-188; 2003.
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