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If the Milk thistle is causing diarrhea, it may be wise not to increase the dose until you stop having diarrhea. Milk thistle promotes the health of the liver. The liver produces bile, and bile has a laxative action. If there is not some other cause of diarrhea, it should normalize as time goes on.
Apart from that, you could have 1 gram (1000 mg) 3 times a day of Milk Thistle.
You could take even larger doses of Milk thistle, but it would be expensive.