Although milk thistle seems to be generally regarded as safe, there are some caveats, as listed in this article which discusses a range of supplements used by people with diabetes:-
http://spectrum.diabetesjournals.org/content/22/4/206
"Milk thistle is thought to be an insulin sensitizer.
68,
69,
72 Adverse effects may include gastrointestinal upset and cross-allergic reactions with members of the daisy and marigold family, including ragweed and chrysanthemums.
7,
68 Milk thistle may have estrogenic effects, and thus women with breast or uterine cancer should avoid its use. Interestingly, milk thistle may inhibit beta glucuronidase and thus increase clearance of administered estrogens.
7 It may inhibit certain isoenzymes in the cytochrome P450 system, such as CYP 2C9, and subsequently increase serum concentrations of warfarin. It may also affect glucuronidation and thus affect serum concentrations of certain statins, anticonvulsants, and benzodiazepines.
7
The dose of milk thistle for liver disease and in the diabetes studies discussed is 200 mg three times daily. Milk thistle extract should be standardized to contain 70% silymarin (140 mg silymarin).
68 Preparations that contain phosphatidylcholine may be dosed at 100 mg/day because phosphatidylcholine enhances oral absorption.
68"