According to an advisory bulletin published late last month from the Iowa Alcoholic Beverages Division, copper mugs may be poisoning drinkers.
Food experts have long known not to use copper, or copper-related pots and pans for cooking acidic recipes such as tomato sauce.
The Iowa advisory says the state has adopted the US Food and Drug Administration's model Food Code, "which prohibits copper from coming into direct contact with foods that have a pH below 6.0".
The pH of a traditional Moscow Mule - which is made with vodka, ginger beer and lime juice - is well below 6.0, according to the bulletin. Examples of these acidic foods include fruit juice, wine, vinegar - and Moscow Mules.
It's the shiny copper mug in which the cocktail is typically served. Although the copper mugs may look stunning, they aren't necessarily healthy when used to house drinks that have an acidity of a pH below 6.0.
Copper is commonly used for kitchenware like pots and pans, but it can be unsafe. Copper poisoning can cause people to experience nausea, vomiting, abdominal pain and liver failure.
Not all copper mugs, however, are poisonous.
Not to worry mule fans, many bars and restaurants that serve mules use mugs that are lined on the interior with another metal, such as nickel or stainless steel.