The tincture of bison grass found in Zubrowka is prohibited as a food additive by the Food and Drug Administration because it contains coumarin, which showed hepatotoxic effects in rats and has a blood thinning effect. Importation of Zubrowka was banned in 1978 by the Bureau of Alcohol, Tobacco and Firearms. Canada has no similar regulations on coumarin, so the alcoholic drink is legal there.
When produced according to traditional methods (between one and two kilograms of grass per thousand litres of alcohol), Zubrowka contains approximately 12 milligrams of coumarin per litre. In 1999, distilleries that were not connected with the Polish brand introduced lower quality reformulated versions of the product, sometimes using artificial flavours and colors, with the emblematic blade of grass in every bottle but "neutralised" so as to be coumarin-free. In 2011 the American licensee of the Polish company worked with Remy Cointreau to introduce a new American formulation, which they called "Zu".
In addition to the Coumarin problem, American authorities determined that the trademark on Zubroowka brand was diluted and unenforceable, as it was a generic name, like "Aspirin".