A delicious moscow mule cocktail with vodka, ginger beer, lime juice and ice.

The name might not be as catchy as ‘Trojan Horse’, but this humble cocktail might be making your customers unwell. The Moscow Mule is one of the simplest cocktails to make, but has a reputation as one of the tastiest and most refreshing, and is widely credited for introducing vodka to the American market at a time of tension between the US and Soviet Union. Tradition dictates that the concoction be served in a copper mug, dating back to the early 40’s when the bartender who invented it sent a Polaroid image of the cocktail in a copper mug he had on hand to a colleague showing him how it should look.

However, the combination of lime juice, ginger beer and vodka means that the Moscow Mule generally has a pH of around 6, meaning it is slightly acidic. This is an issue as the copper can start dissolving into the drink. Copper in solution is considered toxic at concentrations above 1 mg/L. Symptoms of copper poisoning include vomiting, abdominal pain, diarrhoea, and yellow skin or jaundice. As some of these symptoms are usually associated with alcohol consumption, most wouldn’t think twice – however, long-term effects like liver damage can occur if copper-tainted Mule consumption continues over a long period of time.

In the USA, the state of Iowa has come down hard on Moscow Mule mugs. “The recent popularity of Moscow Mules, an alcoholic cocktail typically served in a copper mug, has led to inquiries regarding the safe use of copper mugs and this beverage,” Iowa’s Alcoholic Beverages Division said in a statement. “The use of copper and copper alloys as a food contact surface is limited in Iowa.”

The U.S. Food and Drug Administration 2013 Food Code states that copper and copper alloys such as brass "may not be used in contact with a food that has a pH below 6 such as vinegar, fruit juice, or wine or for a fitting or tubing installed between a backflow prevention device and a carbonator."

All is not lost – the Moscow Mule can still look trendy without the potential for life threatening conditions. Simply make sure that the copper mug is lined with a food safe material such as nickel or stainless steel or, alternatively, just serve it in a glass.