Sadly, qat does have its downside. According to a Yemeni friend, achieving the latter rather than the former is a matter of learning to recognise good quality leaves. What little medical research there is on the long-term health effects suggests that qat may cause mouth cancers, high blood pressure and heart attacks, and may also rot the teeth. Loss of teeth does not put off many chewers. In Yemen, those who have lost their teeth can buy gadgets to mash up the leaves before for popping them into the cheek.
Constipation is another possible side-effect.
A more serious worry is the pesticide sprays used by some qat growers to maximise their crop, because in Yemen there is rarely enough rain to wash away the chemicals before the leaves go to market. For farmers, qat is at least five times more profitable than other crops and produces income throughout the year. Qat has replaced other crops that might have been exported or used to reduce the need for imported food. Some Yemenis spend well over half their income on qat. This, undoubtedly, deprives many families of basic necessities, but it's also an effective way of redistributing what little there is of the nation's wealth from the cities to the countryside.