By Simon Cotton, University of Birmingham
In his humorous novel Black Mischief, the 20th century English writer Evelyn Waugh wrote "Mahmud el Khali bin Sai'ud sat among his kinsmen, moodily browsing over his lapful of khat".
Of course, Waugh wasn't referring to feline friends but to the green leaves of an evergreen shrub (Catha edulis) that grows in East Africa and now the subject of controversy over the decision by Home Secretary Theresa May to categorise it as a Class C illegal drug against expert advice.
A high not banned by the Koran
To some, khat, variously spelled cott or qat, is known as "the flower of paradise". The leaves are chewed by many people in countries like Yemen, Somalia and Ethiopia, in much the same way that coca leaves are chewed in South America.
It's a social drug used by millions of people, and in Muslim countries it offers a high that is not banned by the Koran. It's especially important in the Yemen, where up to 90% of men are estimated to be regular consumers. Chewing sessions normally begin after lunch and go on until dusk.
The main active ingredient is a molecule called cathinone, along with a similar molecule called cathine (not to be confused with caffeine). Cathinone has a similar chemical structure to amphetamine but with a few differences. It's a stimulant, and some people have compared the effect of its consumption with drinking strong coffee.
It's thought to be a relatively safe drug, but can produce symptoms similar to amphetamine: people become very alert, cannot sleep, and may have heart palpitations. A small case-control study of 100 people showed heavy khat chewers (chewing for over six hours) had nearly a 40-fold increased risk of a heart attack - though cigarette smoking was also a factor.
Concerns have also been raised that chewing khat contributes to economic problems - both for individuals and for the state - because of lost working hours and because khat cultivation uses huge quantities of water.
Khat in the UK
Together with coffee, khat is an important cash crop in the Yemen and much of it is exported, not just to neighbouring Ethiopia and Somalia but further afield. Once the leaves are picked, the cathinone is oxidised quite fast (within a couple of days) so only fresh leaves are active. Because of this, it is air-freighted into destinations like London for sale to expatriate communities.
While khat is banned in other countries, including France, Germany, the Netherlands and the US, it's legal in the UK. And around 60 tonnes of it are imported into the UK each week, which has led to claims of its abuse by immigrant communities, and especially Somalis. It's also been claimed that much of the UK import is surreptitiously re-exported to Europe and the US.
Now for the science bit
Cathinone and amphetamine are both phenylethylamines, which means they have a similar structure that consists of a short chain of carbon atoms that have an amine group at one end - a group of atoms related to ammonia but where some of its hydrogen atoms have been replaced - and a phenyl ring at the other end, which is six carbon atoms bonded together in a hexagonal shape.
But they differ because amphetamine has something called a CH2 group in one part of its carbon chain, where cathinone has a carbonyl group - one carbon atom double-bonded to an oxygen atom.
Playing around with the structures and positions of these atoms produces other chemical compounds.
For example, if you replace a hydrogen atom in the amine group in cathinone with a methyl group, you have a molecule called methcathinone. Methcathinone is totally synthetic and doesn't occur in nature. It can be made from ephedrine - used as an appetite suppressant and as a performance enhancer in sport. It also produces amphetamine-like symptoms when abused, notably in Russia. Ephedrine can also be used to make the drug crystal meth.
You probably haven't heard of methcathinone, but you will have heard of mephedrone - or meow-meow as it was later nicknamed. This has a structure similar to methcathinone, but with an extra methyl group attached to its benzene ring. It was first made around 1929 but languished in obscurity for most of that time until it was rediscovered ten years ago and marketed as a legal high.
Mephedrone came to be seen as an alternative to taking ecstasy or cocaine, and took off worldwide around 2008 and especially once it could be purchased over the internet and because ecstasy supplies became unreliable.
But within a short while, doctors began to see patients who exhibited symptoms including chest pains, high blood pressure and a racing heart. It seemingly worked like amphetamines, boosting levels of neurotransmitter molecules like dopamine and noradrenaline.
Mephedrone was made illegal in 2010 but even before its safety was dubious; unlike the kind of prescription drugs you get from your doctor, it didn't come in packets with printed instructions telling people how much to take.
The potential dangers were seized on by the press and the wildest story was that someone ripped off their own scrotum after taking it - though this story later turned out to be the result of an online joke.
Why is khat different?
May says her decision to ban khat because of prominent health and social harms and to put Britain in line with most of the rest of Europe and to avoid the UK becoming a "single, regional hub" for illegal onward trafficking. The government's own Advisory Council on the Misuse of Drugs (ACMD) said there wasn't enough evidence.
Despite some chemical similarities with khat, mephedrone doesn't occur naturally. Mephedrone is a man-made stimulant that suddenly became flavour of the month and was taken by many thousands of people who had no clear idea of its safety. Within two years of the first reports of fatalities towards the end of 2008 - though none conclusively blamed on mephedrone by coroners - it had been banned in the European Union, as well as the US and many other countries.
Khat on the other hand is a plant-sourced stimulant that has been taken in some parts of the world for hundreds of years.
The case for better scientific literacy should be made.