The president's gesture - intended to set an example to the nation - comes amid pressure from two normally incompatible elements: religious radicals who object to qat on moral grounds, and secular modernisers who blame many of Yemen's ills on the amphetamine-like properties of catha edulis - "this evil plant", as the Yemen Times called it.

Qat, however, is a national institution. Yemenis have been chewing for more than 700 years and all previous attempts to clamp down on the country's favourite substance have ultimately failed.

At first sight, the case against qat is both simple and obvious. Yemenis typically spend 4-6 hours a day buying and chewing the leaves when, according to the anti-qat lobby, they could be doing productive work - hence the dire state of the economy. Some spend well over half their income on the habit. Qat keeps people awake, so chewing sessions, which last for several hours, start in the afternoon to allow the effects to wear off before bed. As a result, virtually the entire civil service shuts down at lunchtime.

To meet the ever-growing demand, one-third of Yemen's agriculture is now devoted to a crop with no nutritional value, and irrigating it consumes scarce water supplies. Qat has supplanted other crops which could be exported or used to reduce the need for imported food.

In the past, qat was regarded as an occasional luxury rather than a daily necessity. Consumption among city-dwellers increased in the 1970s with the development of roads. The plant grows best at an altitude of 3,000-6,000 feet and good transport from the growing areas to the cities is essential because the leaves rapidly lose their potency after cutting.

Consumption in southern Yemen increased dramatically after unification in 1990. The previous Marxist regime had forbidden qat growing on state-owned land and chewing was allowed only at weekends.

The Marxists believed that qat made people lazy - though Yemeni students often use it to help them work harder at examination time. Defenders of qat reject the argument that chewing equals time-wasting. Qat sessions are for social bonding, for exchanging information and ideas, and arranging business deals, they say.

Although qat is frowned upon by radical Islamists, it is not specifically forbidden in the Qur'an, and some Yemeni religious leaders are enthusiastic chewers. Devout Yemenis reputedly used it in the past to remain alert during all-night prayer vigils.

For the growers, qat is five times more profitable than coffee and produces income throughout the year. The qat trade has brought money to remote villages and made them more accessible; in some places it has funded irrigation projects which would otherwise have been uneconomic.

The long-term health effects of qat are uncertain. Depending on one's prejudices, it either rots the teeth or protects them against decay, serves as an aphrodisiac or causes impotence. One proven side-efffect is constipation: during a short-lived ban in the south some years ago, sales of laxatives fell by 90%.

Psychologically, according to one textbook, qat-chewing can also induce "fantasies of personal supremacy" - a claim which may be put to the test in the forthcoming presidential election.