Grab a bitter leaf and chew. Then take another and another, letting the wad rest in your cheek. Soon you'll feel less hungry, more alert, a little euphoric. That's qat (pronounced cot, often spelled khat), a stimulant used for centuries in Yemen and Africa's Horn by laborers for energy and by men to while away afternoons. Today, with increased urbanism, easier access to cash, and relaxed social mores, it's taking deeper root. "People chew it in the early morning, on the street," says psychologist Michael Odenwald. "Children and breast-feeding women chew it."

Qat's fanning out too, flown daily to African and Yemeni expats in Europe, Australia, and North America while also entering Uganda and Rwanda. With greater demand and better transport- which gets qat to market in 48 hours, while it's still fresh and potent-farmers are planting more of the profitable, easy-to-grow crop. In Yemen, the cultivated area has increased more than tenfold since 1970; in Ethiopia, qat has become a top foreign-exchange earner.

The plant's spread raises concerns, though. In Yemen, it's irrigated from shrinking aquifers. In Somalia, Odenwald has seen abuse linked to mental-health problems. And in the West, countries have debated whether to leave the leaf legal, like tobacco, or ban it, like marijuana. Qat will get you arrested in the U.S., Canada, and much of Europe. In the U.K., for now, it's perfectly fine. -Karen E. Lange

Photo: A seller in Hargeysa, Somaliland, offers enough qat for an afternoon of chewing. The price: $10.

Photograph by Pascal Maitre