By 4 in the afternoon, most men walking the streets of Sana'a are high, or about to get high — not on any sort of manufactured narcotics, but on khat, a shrub whose young leaves contain a compound with effects similar to those of amphetamines. Khat is popular in many countries of the Arabian peninsula and the Horn of Africa, but in Yemen it's a full-blown national addiction. As much as 90% of men and 1 in 4 women in Yemen are estimated to chew the leaves, storing a wad in one cheek as the khat slowly breaks down into the saliva and enters the bloodstream. The newcomer to Yemen's ancient capital can't miss the spectacle of almost an entire adult population presenting cheeks bulging with cud, leaving behind green confetti of discarded leaves and branches.For its many devotees, khat is a social lubricant on a par with coffee or alcohol in the West. Indeed, because chewing the leaf isn't forbidden by Islam, "khat is alcohol for Muslims," says Yahya Amma, the head merchant at the Agriculture Suq, one of the largest khat markets in the city. "You can chew it and still go to prayers." The leaf's energy-boosting and hunger-numbing properties help university students focus on their homework, allows underpaid laborers to work without meals and, according to local lore, offers the same help to impotent men that Westerners seek in Viagra. Evening khat ceremonies — regular salon gatherings (usually only of men) to chew and chat about matters great and small — are the country's basic form of socializing.But khat's detractors say the leaf is destroying Yemen. At around $5 for a bag (the amount typically consumed by a single regular user in a day) it's an expensive habit in a country where about 45% of the population lives below the poverty line. (Most families spend more money on khat than on food, according to government figures.) A khat-addled public is more inclined to complacency about the failings of the government, khat ceremonies reinforce the exclusion of women from power and, as is obvious to anyone finding a government office nearly empty on a weekday morning, khat is keeping the country awake well past its bedtime."You sit up discussing all your problems and think you've solved everything, but in fact you haven't done anything in the last four hours, because you've just been chewing khat and all your problems actually got worse," says Adel al-Shujaa, a professor of political science at Sana'a University and the head of the Yemen Without Khat Association. Plus, he says, "all the decisions you've made are bad because you've made them while on khat."But the worst thing about khat may be that it is sucking Yemen dry.The plant thrives in the high hill country outside Sana'a, where nearly every patch of irrigated land is covered in khat. Unlike coffee, which Yemenis claim was first cultivated here, khat is easy to grow and harvest. And though cultivating and dealing the leaf doesn't generate the kind of instant wealth associated with growing poppies in Afghanistan or coca in Colombia, it certainly provides a steadier income than growing vegetables does — that's why nearly all of the country's arable land is devoted to khat. And khat needs a lot of water, which is scarce in Yemen.Khat fields are typically flooded twice a month, consuming about 30% of the country's water — most of which is pumped from underground aquifers filled thousands of years ago, and replenished only very slowly by the occasional rainfall that seeps through the layers of soil and rock. A recent explosion of khat cultivation has drawn water levels down to the point where they are no longer being replenished. The option of pumping desalinated water over long pipelines from coastal plants is too expensive for such a poor country. Yemen is in real danger of becoming the world's first country to run out of water."I tell UNHCR that they should start buying tents [for the communities that would be forced to move in search of drinking water]," says Michael Klingler, a hydrologist and the local director of GTZ, the German government's technical-assistance team, which is advising Yemen on water-management issues.A massive drought — accelerated by khat cultivation — and the resultant population displacement could have a devastating impact in one of the most fragile countries in the Middle East. A separatist insurgency in the south is threatening to break the country apart, while pirates from Somalia are menacing the coast. Al-Qaeda, meanwhile, has long seen the lawless tribal lands in the northern mountains as a potential sanctuary.Quitting khat would double the amount of household water available, says Klingler, but that may only slow the onset of crisis. The hydrologist argues that Yemen needs to revert to consuming only as much water as it collects from rains — and to import most of its food from abroad.Despite the danger, Yemen isn't about to go cold turkey anytime soon. Not only are most of the country's leaders landowners deeply involved in khat production, the leaf may be one of the few things still holding Yemen together. Says Ashraf Al-Eryani, one of GTZ's local program officers, "Khat plays a big role in keeping people calm, and keeping them off the streets. But it's also delaying change. It's hard to convince people to act now."