Research UK stated: "We need a lot more research with reliable clinical trials before we can be sure that milk thistle will play any part in treating or preventing cancers." Use of milk thistle may cause stomach upset and produce allergic reactions in some people. Toxicity Milk thistle based supplements have been measured to have the highest mycotoxin concentrations of up to 37 mg/kg when compared to various plant-based dietary supplements. Animal toxicity Because of nitrate content, the plant has been found to be toxic to cattle and sheep. When potassium nitrate is eaten by ruminants, the bacteria in the animal's stomach breaks the chemical down, producing nitrite ions. Nitrite ions then combine with hemoglobin to produce methemoglobin, blocking the transport of oxygen. The result is a form of oxygen deprivation. References Further reading UC Davis profile for blessed thistle marianum Medicinal plants Christian folklore CYP3A4 inhibitors Flora of Lebanon Flora of Israel Flora of Palestine (region) Flora of Europe Edible plants