Copper Drinking Vessels

Background:

Unlined copper drinking vessels such as Moscow Mule mugs, copper shakers, and punch bowls can leach copper into cocktails if they are acidic. Most cocktails with citrus are acidic. The US FDA recommends against allowing any acidic (pH less than 6) food or liquid to come into contact with copper. 

In 2017, the state of Iowa issued a notice, following the US Food and Drug Administration guidelines, that acidic drinks like the Moscow Mule were not to be served from unlined copper vessels. 

The website Snopes.com does a great job covering this issue [link] in their article "Can Drinking Cocktails From a Copper Mug Cause Poisoning?" Much of the information on this page comes from that article.

Health Concerns:

Copper poisoning is the concern, as copper can be leached out of the container into the beverage. The amount of copper leached would be due to the acidity level of the beverage and the amount of time time it spends in contact with copper. 

A doctor cited in the Snopes article estimated there would be low risk from drinking, "three to five acidic cocktails from the same copper mug over the course of several hours." [link]

To reduce copper leaching, one should not store acidic beverages in unlined copper for any significant length of time. 

Legal Concerns:

The United States Food and Drug Administration's Food Code [link 2017] states:

4-101.14 Copper, Use Limitation.
(A) Except as specified in ¶ (B) of this section, 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. 

(B) Copper and copper alloys may be used in contact with beer
brewing ingredients that have a pH below 6 in the prefermentation
and fermentation steps of a beer brewing operation such as a
brewpub or microbrewery.

The FDA Food Code is not a law in itself but is is offered as a guideline to be followed by state/regional authorities, who may enforce it. The FDA states: 

The Food Code is a model for safeguarding public health and ensuring food is
unadulterated and honestly presented when offered to the consumer. It represents
FDA's best advice for a uniform system of provisions that address the safety and
protection of food offered at retail and in food service.

This model is offered for adoption by local, state, and federal governmental
jurisdictions for administration by the various departments, agencies, bureaus,
divisions, and other units within each jurisdiction that have been delegated
compliance responsibilities for food service, retail food stores, or food vending
operations. Alternatives that offer an equivalent level of public health protection to
ensure that food at retail and foodservice is safe are recognized in this model.

Alternatives: 

The good news is that many modern copper vessels are either lined on the inside with another metal such as stainless steel, or coated on the inside with a food-safe lacquer to prevent beverage contact with copper. When purchasing copper vessels, be sure to see if they are protected from copper contact. 

Further Reading

"Can Drinking Cocktails From a Copper Mug Cause Poisoning?" Snopes [link]