Gigi Hadid, meet Brittney Griner. At first glance the famous 28-year-old American model may appear to have little in common with the lanky 32-year-old professional basketball player, but as of this month the two women have a link: both were arrested in foreign countries on drug-related charges. 

Ms. Hadid was arrested on July 10 at the Cayman Islands’ Owen Roberts International Airport after customs officials there found marijuana in her luggage. Last year Ms. Griner was detained at Moscow’s Sheremetyevo International Airport after Russian customs officers found about a gram of hash oil in her belongings. 

The similarities do not end there. Ms. Griner claimed that she had been prescribed medical cannabis in Arizona — aboveboard in the Grand Canyon State but categorically  illegal in Russia. Ms. Hadid’s representative said in a statement that the model was traveling with marijuana purchased legally in New York “with a medical license.” It is not clear for which medicinal purposes the two women intended to use their transported substances, nor was the legality of marijuana possession for any kind of use in the Cayman Islands immediately clear. 

Ms. Hadid had a far easier time with authorities in the Caribbean than did Ms. Griner with those at Moscow.  According to the local newspaper Cayman Marl Road, Ms. Hadid and her friend were initially released on bail following their arrest. The pair were then formally charged on July 12 during a court appearance, where they pleaded guilty and were both fined $1,000. By July 14, Ms. Hadid was posting pictures of herself with friends on the beach with the perky caption, “All’s well that ends well.”

Yet not every American is a wealthy and well-connected supermodel. Far from setting an example, Ms. Hadid — who has sometimes drawn criticism for her outspoken views seen as unfashionably favorable to Palestinian terrorist groups —  has taken a cavalier approach to a serious legal infraction that most people, as Ms. Griner’s drama demonstrated, can ill afford.

In Ms. Griner’s case, the Russians were quick to mete out their version of justice that against the backdrop of badly frayed American-Russian relations was widely perceived as politicized. After she pleaded guilty to charges, Ms. Griner was fined the equivalent of more than $16,000 and sentenced to nine years in prison. She spent several tense weeks in the Russian prison system before being freed in a prisoner exchange. 

The political dimensions notwithstanding, what Ms. Hadid and Ms. Griner both failed to understand is that just because something is legal in the United States of America, once abroad it is not necessarily so. The gradual legalization of marijuana on our shores has not been duplicated globally, and that can cause problems for Americans who naively assume that the rules that apply at home automatically hold overseas. They do not.

Ms. Hadid may be too young to have seen or understood the significance of a movie like “Midnight Express,” the 1978 film that presents a lightly fictionalized account of an American citizen who was jailed in Turkey in miserable conditions for four years after he was caught smuggling hashish. The film may not have deterred illicit drug use in America, but the damage it did to Turkey’s reputation as a country safe for tourism was substantial. It stands as a cinematic symbol of prenez garde — pay attention — for travelers to this day. 

Today in Communist China and Iran but also in Vietnam, Thailand, Malaysia, and elsewhere there are foreigners who face lengthy prison sentences or worse for drug smuggling. In 2015, a group of convicted drug smugglers, among which were a Brazilian and two Australians, were put to death in Indonesia.

A big potential pitfall right now, particularly with cannabis, is that what a traveler might perceive as simply carrying a substance that is legal at home could in a foreign country be considered as an act of smuggling. In some places laws on the books are particularly draconian. In Singapore, there is a Misuse of Drugs Act that holds that if one is found in possession of even half an ounce of cannabis, the offender will be presumed a drug smuggler. If 18 ounces or more are found, the penalty for the crime can be death. 

In other countries like Egypt tourists have even been arrested and jailed for carrying certain kinds and quantities of prescription medication in their luggage. 

Many Americans are taking to the skies this summer for vacation, and many will be departing from states where marijuana in certain quantities and circumstances is now legal. To avoid the kinds of problems our fellow citizens have faced in the Caymans and certain less enlightened countries in other latitudes, here is the Sun’s easy rule of summer travel to follow: Flush it down before you leave town.