An advisory bulletin has been issued in the US, warning of potential health risks from serving Moscow Mules in copper mugs. Acidic liquids (like the Moscow Mule) can cause the copper from the mug to leach into your drink. This can ruin the fun at happy hour by giving you food poisoning. Oh. The warning states that "high concentrations of copper are poisonous and have caused foodborne illness."
Turns out, it's all to do with the pH of the cocktail. “The federal Food and Drug Administration’s Model Food Code […] prohibits copper from coming into direct contact with foods that have a pH below 6.0,” says the statement. “Examples of foods with a pH below 6.0 include vinegar, fruit juice, or wine. The pH of a traditional Moscow Mule is well below 6.0. This means that copper mugs that have a copper interior may not be used with this beverage.”
But, before you go binning all your fancy mugs – check the label. Their statement says, “copper mugs lined on the interior with another metal, such as nickel or stainless steel, are allowed to be used and are widely available.” So there’s a good chance your mug could be a-okay for happy hour. Check it, then cheers!