Walk down any major street in Yemen in the afternoon or evening, and you'll see men with bulging cheeks, chewing qat leaves; their constituents, cathinone and cathine, produce a high. Qat — or Catha edulis — is cultivated in the Horn of Africa as well. But in Yemen, buffeted by fierce government-tribal clashes in the north, renewed secessionist strength in the south and dwindling oil revenues, the qat shrub is just about holding the Arab world's poorest country together.

Qat chewing occurs almost everywhere in Yemen, except tourist hotels (one in Aden greets visitors with a sign, "Guns and qat are not allowed"). Many private homes have a comfortable, well-ventilated room, or diwan, set aside for the purpose. But it is at street level that the pervasiveness and tempo of the activity can best be appreciated, in the qat markets, or drifting amid those chilling out on it or consuming it during their workday as a taxi driver or an attendant for kids' camel rides at a park, or just shopping for fruit and vegetables.

If it is a ritualised activity, it is a seamless one, like taking coffee after a meal is for a westerner.

Partaking of this natural amphetamine is not prohibited in the Qur'an, and the jury remains out on whether it is addictive or harmful. Accepted in Yemen, it is not in other Arab countries; and while legal in the UK and much of Europe, it is banned in France, Norway, Sweden, the US and Canada.