There may be no other alcoholic cocktail as immediately recognizable as a Moscow mule. It’s not the actual beverage that makes it so iconic, but the flashy copper mug that is almost always used to serve it. Now, health experts are warning that those fancy vessels might actually be seriously dangerous thanks to the reaction between the drink’s ingredients and the metal of the mug. 

The notice, which comes from the Alcoholic Beverages Division of Iowa’s food and beverage regulatory body, points out that the traditional Moscow mule is hazardous if served in direct contact with copper. Using the FDA’s guidance on prohibiting the serving of foods below a PH of 6.0 in contact with copper, the group issued a stern warning to bars and restaurants.

“The pH of a traditional Moscow Mule is well below 6.0,” the bulletin reads. “This means that copper mugs that have a copper interior may not be used with this beverage. However, copper mugs lined on the interior with another metal, such as nickel or stainless steel, are allowed to be used and are widely available.”

When filled with an acidic liquid like a Moscow mule, the risk of copper poisoning from an unlined copper mug is present. Symptoms include vomiting and diarrhea, which could easily be confused with the typical result of having one too many cocktails.

It’s worth noting that while many different companies produce copper mugs with an inner lining, there’s also many that don’t. If you search for mule mugs on Amazon, many of the top sellers are unlined pure copper.

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