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LA PAZ, Bolivia — Pope Francis enjoyed an in-flight tea made partially with coca leaves — illegal in the United States — to prevent altitude sickness during a visit to the world’s highest administrative capital.

Sitting at an altitude of 13,000 feet, La Paz has air 30 percent thinner than at sea level and the reduced oxygen level can cause fatigue, headaches and shortness of breath. That was a real concern for Francis, who has had part of one lung removed as a child due to an infection.

Francis — Latin America's first pope — is on a tour of the region.

Staff on the Bolivian Airlines plane from Quito, Ecuador, on Wednesday offered journalists a traditional remedy of coca tea to reduce the effect of altitude sickness.

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A Peruvian farmer collects coca leaves. Rodrigo Abd / AP

A flight attendant told NBC News that the pontiff was instead served Trimate — a tea mix of coca, chamomile and anise seeds.

Coca is known throughout the world for its psychoactive alkaloid, cocaine. But because the leaves have very low alkaloid content, they are used as a mild stimulant — unlike the high produced by chemical cocaine.

Importing the leaves into the United States is prohibited — the Coca-Cola company has an exemption, but removes the cocaine traces from the plant during its manufacturing process. However, in the Andes the leaves have been chewed and brewed as a mild stimulant for centuries.

To help Francis cope with the altitude sickness, Vatican officials also planned a stopover of only four hours in La Paz before his entourage moved to the eastern city of Santa Cruz de La Sierra.

But with typical spontaneity, Francis prolonged his stay by more than an hour, to the joy of the many faithful who came to greet him.

Several reporters waiting for the pope at the capital’s El Alto airport, the highest in the world, began to suffer altitude sickness and some had to be given oxygen through masks.

One further headache for the Vatican came during the traditional exchange of gifts with Bolivian President Evo Morales.

He gave the pope a crucifix carved into a wooden hammer and sickle, the Communist symbol uniting labor and peasants.

Bolivian President Evo Morales exchanges gifts with Pope Francis in La Paz. OSSERVATORE ROMANO / AFP - Getty Images

Pope Francis was branded a Marxist by American radio talk show host Rush Limbaugh, a charge the pontiff strongly denied.