It's the shiny copper mug in which the cocktail is typically served. Because the pH level is so low in mule, the copper may leach into the drink and cause illness. It's not the actual beverage that makes it so iconic, but the flashy copper mug that is nearly always used to serve it.
Food and Drug Administration (FDA) guidelines - adopted by multiple states, including Iowa - prohibit copper from coming into direct contact with foods like vinegar, fruit juice or wine that have a pH below 6.0. The cocktail drink only uses a few ingredients namely vodka, lime, ginger beer and ice but it is often served in copper mugs. The long-term effects of copper poisoning also include liver damage or cirrhosis. The symptoms of copper poisoning include vomiting, abdominal pain, diarrhea, or jaundice (yellowing of the skin and eyes). According to food poisoning attorney Bill Marler, it would take years to ingest a risky amount of copper by drink drinking Moscow Mules alone.
And, if you're drinking Moscow Mules at every meal, everyday, copper poisoning may be the least of your problems. A different interior should line the inside of the cup, such as stainless steel and nickel.
"You're at more risk for alcohol consumption", Marler said.