When President Ali Abdallah Salih came clean about his drug habit during an election campaign, nobody here really cared what he did, how much or how frequently. But when the Yemeni president renounced the use of khat in hopes that the rest of the country would break tradition and follow his lead--now that was news.

Salih said he would fill his time with exercise and computer lessons instead of chewing the bitter green leaf that is Yemen's drug of choice. His appeal, launched over the summer, sent a stir through the local markets and roused curiosity about what the president expected people to do with their time if they swore off khat and its mildly addictive stimulant effect.

"Everybody knows [khat] is of no good use," said a dealer in one of the seven open-air markets where Yemeni men gather each afternoon to buy a daily supply that, chewed for a long time, gives them a buzz similar to what large quantities of caffeine might produce. "But there is nothing else to entertain us."

"It would be a good idea for all of Yemen, but I don't think he'll be able to," said another dealer, Muhammad Ali Darnajay, who supports Salih's call for abstinence. Historically, the odds are not good for the president. Khat has been chewed in Yemen for perhaps half a millennium, beating efforts to tax it or eradicate it in the formerly communist southern part of the country. It has supplanted other cash crops like coffee to occupy perhaps as much as two-thirds of the arable land. One estimate is that khat accounts for as much as 20 percent of Yemen's $40 billion gross domestic product.

"I tried to give it up but for three days I felt depressed and sleepy and lazy," said Amin Azury, a young user from the village of Shibam, an hour's drive from Sanaa. "You can chew khat and stand on a building 12 stories high. . . . I have so much power that I could be a leading force for Islam."

Bought in freshly picked bundles for each afternoon's use, the leaves are chewed in a large, continuously replenished wad over several hours, until enough of the juice has been ingested for the plant's natural amphetamine to take effect. Somewhat like an intense coffee high, it made one man chew his fingernails until they bled. The leaf is commonly associated with heightened feelings of personal prowess and formless, poetic creativity.

"Khat," said one ranking politician, "is in Yemen to stay."

Salih, nevertheless, declared he was giving it up and urged his ministers and as many Yemenis as possible to do likewise. For the president, it was a swap of traditional habits for more up to date ones that he would like to see the country adopt in other areas too.

With a new constitution and multiparty system, Yemen recently had its first contested presidential election, and Salih said repeatedly during the campaign that he wanted the country to shed its lawless image--of civil war, tribal justice and tourist kidnappings--to become a model the rest of the Middle East would follow. A place of relatively mild climate, home of the ancient Sabean civilization, existing in the shadow of Saudi Arabia but proudly distinct in its culture and politics, Yemen is the greenest country in the Persian Gulf and arguably the most violent. Damping the use of khat would be as much a part of the country's make-over as Salih's efforts to ban guns from the streets of major cities and break the penchant of Yemeni men to use AK-47 assault rifles as a fashion accessory.

His decision also reflects an opinion commonly held among local users, teetotalers and outsiders alike: That Yemen's khat habit is both unhealthy and economically ruinous, a premise the government is analyzing in a major new study meant to quantify the effect the drug is having on this country of 16 million.

Many of the conclusions are already suspected. Cultivation of the leaf is so widespread it has displaced food plants, making one of the Middle East's more fertile countries ever more dependent on agricultural imports. Millions of dollars change hands daily in a trade that is legal but draining in one of the region's poorest countries.

The health effects have also been hinted at in various medical reports that linked the hours of daily chewing to stomach, oral and prostate problems, insomnia, temporary schizophrenia and other ailments. The added toll of chemical fertilizers and pesticides is a new subject of concern: Few Yemenis wash the leaves before chewing.

This is not the only nation where khat is used. It is also common in the nearby African nations of Ethiopia and Somalia, and is even available in some U.S. cities. But nowhere has the practice become so culturally rooted. Most Yemeni homes have a room arranged with upholstered seats along the walls for hosting khat sessions, affairs that linger from midafternoon sometimes until late in the evening, and which can be the scene of business deals and animated political debates as well as glassy-eyed stupor.

During the elections voters carried khat to the polling places with them; motorists and pedestrians by midafternoon commonly have a bulging wad of khat leaves in their mouths; on one flight into Yemen, airline staff even offered khat to a western visitor as a welcoming present. In one token gesture, the country recently banned the use of the drug by on-duty security officers.

All of which points to the difficulty Salih and others expect in taming khat use.

Although not considered as physically addicting as cocaine or heroin, khat's active agent, the amphetamine cathinone, places it in the same category as those hard drugs under U.S. law. Users and medical reports say a strong psychological dependence can develop.

Accounts collected on one Yemeni Web site show the extremes. Some users reported a sense of timelessness and enhanced personal power, while others said the drug can also drive them into neurotic fits. One man reported repeatedly fixing his household appliances. While folklore touts it as a male aphrodisiac, one survey concluded that "most women disagree," because khat tends to kill almost all appetites. Users often skip the evening meal.

CAPTION: Yemenis chew khat, a popular and mildly addictive stimulant, as they stand in line with their voter registration cards during Yemen's first contested election.

CAPTION: Yemeni President Ali Abdallah Salih has said he will stop using khat and wants others to do the same.