http://en.wikipedia.org/wiki/Coumarin
Coumarin is a chemical compound (benzopyrone); a toxin found in many plants, notably in high concentration in the tonka bean, woodruff, and bison grass. It has a sweet scent, readily recognised as the scent of newly-mown hay, and has been used in perfumes since 1882. It has clinical medical value as the precursor for several anticoagulants, notably warfarin, and is used as a gain medium in some dye lasers.
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Coumarin toxicity
Coumarin is often found in tobacco products and artificial vanilla substitutes, though it has been banned as a food additive in numerous countries since the mid-20th century because it is moderately toxic to the liver and kidneys, with an LD50 of 275 mg/kg - low compared to related compounds. Although only somewhat dangerous to humans, coumarin is a potent rodenticide: rats and other rodents largely metabolize it to 3,4-coumarin epoxide, a toxic compound that can cause internal hemorrhage and death. Humans largely metabolize it to 7-hydroxycoumarin, a compound of lower toxicity.
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So my question would be, WHY would this ingredient be put into exercise supplements? I don't see how this could potentially be productive. I also don't see how a substance "slips through the cracks" and could accidently find its way into proprietary blend.