If you're a HUGE fan of Moscow Mules, and especially if you have your own set of Moscow Mule mugs, this public safety announcement is meant for you.
An advisory warning from Iowa's Alcoholic Beverages Division is reminding people to ensure that, per FDA guidelines, you should avoid putting acidic foods into contact with copper. Their warning specifically highlights the Mule, which is commonly served in an iconic copper mug and is definitely acidic thanks to the alcohol and lime juice found inside.
In fact, most foods are in the acidic range that the Alcoholic Beverages Division warns people of. Most foods have a pH under 6.0, which is the minimum acidity level mentioned in the above bulletin. To go any more acidic (or to a lower pH level) would cause the copper in the mugs to erode into the food, which can cause copper poisoning at high enough levels. While rare, if enough copper is ingested at once, it can potentially cause liver failure and death.
Fortunately, there is a simple solution to prevent this type of poisoning from occurring. As is commonly done with all copper pots, pans, and cookware already, many, though not all, Moscow Mule mugs are lined with another metal like stainless steel or aluminum that blocks corrosion. If you accidentally purchased unlined copper mugs, however, I'd avoid drinking anything other than pure water in them, just to be safe.