Human studies performed with milk thistle seeds indicated little need for concern regarding adverse effects, demonstrating that milk thistle seed extract (silymarin) is safe and well tolerated. It is generally non-toxic, and causes no side effects when administered to adults in a dose range of 200—900 mg/day in two or three divided doses. Higher doses (> 1500 mg/day) could produce minor gastrointestinal disturbances involving a slight laxative effect, which may be due to increased bile secretion and flow. Mild allergic reactions (pruritus, urticaria, arthralgia) are observed, but are rarely severe enough to cause discontinuation. Commonly noted adverse effects, such as bloating, dyspepsia, epigastric pain, flatulence, nausea, irregular stools, and laxation, were observed in 2—10% of patients in a clinical trial. Headaches and dermatological symptoms were also noted (Anonymous, 1999; Barceloux, 2008).
Silymarin was found to be non-toxic in rats and mice after oral doses of 2500 or 5000 mg/kg body weight, producing no unwanted symptoms. Similar reports were also obtained for rabbits and dogs. No evidence of ante- or postnatal toxicity in animals was reported. These data reveal that the acute toxicity of silymarin is very low (Desplaces et al., 1975; Barceloux, 2008).
It was found that silymarin at higher concentrations has an inhibitory effect on both phase I and phase II hepatic drug metabolizing cytochrome enzyme systems. However, the plasma concentrations at therapeutic doses are very much less than that needed for inhibition, so it exhibits no beneficial or harmful drug interactions at normal doses (Barceloux, 2008).
Safety of milk thistle seed in pregnancy and lactation has not been studied in humans. Traditionally considered safe in lactation, no clinical studies have been performed in this respect. Safety in children has also not yet been studied. No known contraindications have been reported (Barceloux, 2008).
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