Mustard oil contains high levels of erucic acid, a fatty acid that has been linked to health issues. Studies have shown that erucic acid can cause myocardial lipidosis (fat accumulation in the heart) and other heart-related problems in animals.
The U.S. Food and Drug Administration (FDA) has not recognized mustard oil as safe for consumption. The FDA has specific standards and regulations concerning food safety, and mustard oil's high erucic acid content exceeds the allowable limits for edible oils in the United States.
In the U.S., mustard oil bottles are typically labeled with a warning that states "For external use only," indicating that it is not intended for cooking or direct consumption.