There are fears that the stimulant khat is contributing to mental health problems within the UK's east African communities. Dil Neiyyar of the BBC's Asian Network reports.
Ahmed Abdillah runs a "khat house" in east London
In a courtyard, two men sitting on a bench are staring intensely at a pond.
The pair appear mesmerised by the gentle and repetitive splash of the water from a small fountain.
This walled oasis in London's East End is the Tower Hamlets branch of the mental health charity Mind.
In one of its many rooms, Abdi Rahman is playing pool with a friend.
He explains why he thinks the stimulant khat is responsible for the loss of his job and subsequent slide into mental illness.
"While I was working as a postman I got depressed and stressed, so I was admitted to hospital, and at the same time I was using khat as well, so they all added up and eventually I ended up mentally ill."
Heavy use can lead to insomnia, high blood pressure, heart problems and impotence
Longer-term risk of developing mouth cancers
Can create feelings of anxiety and aggression, and cause paranoid and psychotic reactions
Can make pre-existing mental health problems worse
Roukiya Omar is one of the workers whose job is to educate the community about the dangers of khat use and encourage people to stop chewing.
She believes the stimulant is moving away from its traditional use as a social pastime because of unemployment and social exclusion. She says it's a growing problem, particularly with women.
"[They are] resorting to khat to seek any comfort from the real problems they are having," she says.
"It's like with any addiction, when people can't handle life they just go and get a drink or go for drugs, so khat is becoming something like that."
About 15 minutes' walk from Mind is a building which was once home to Captain James Cook. Out of its dilapidated basement, Ahmed Abdillah runs a "khat house" where the plant can be bought and consumed.
Today there are three men sitting on the floor.
They all have a tell-tale vacant stare and their clothes have seen better days. One of the men describes how chewing khat can feel.
"It gives a little bit of a high but not as high as a drug or high as alcohol, it just makes you relaxed and comfortable and then you just enjoy yourself."
Ahmed, the owner of the khat house, has no reservations about selling the substance that he says isn't harmful if used in moderation.
"When you do it 24 hours, chewing, you become like an alcoholic... I eat four pieces a day, and no problem for me. When I eat, wake up in the morning, go to work - no problem."
Twice a week, Ahmed drives to Heathrow to pick up a fresh delivery.
Around seven tonnes of khat is flown into Britain every week from Ethiopia, Kenya and Yemen and then sold in cities with large east African communities like London and Birmingham for about £4 a bunch.
The khat plant, Catha edulis, has been chewed by east Africans for hundreds of years and plays a large part in the social lives of both men and women.
A typical session might consist of consuming several bundles.
Cathinone and cathine are the main ingredients of the plant. Both are class C drugs in the UK, but the plant khat itself is not classified and can be bought openly in shops.
Khat is sold openly on the streets in places like Yemen
Cathinone is almost identical to amphetamines and it is this that creates a high. It's known to cause mental health problems like psychosis and depression.
At the Mile End Hospital in Tower Hamlets, consultant psychiatrist Dr Eleni Palazidou treats many people from the Somali, Yemeni and Ethiopian communities, and claims khat is a serious factor in many of these cases.
She says it exacerbates existing mental health problems but believes it can also directly trigger psychosis if used in excess. One of the main problems facing treatment of these effects is habitual usage.
"They may have a new episode triggered off by the use of khat or they may get better from an episode of illness and they chew khat and go back to square one," she says.
"It's difficult to effectively control their illness, because it stimulates those particular chemical systems in the brain that we are trying to control with the medication."
KHAT: LEGAL STATUS
Banned in the US and Canada
Banned in many European countries - Norway, Sweden, Denmark, Finland, Ireland, France, Germany, Switzerland - but not the UK
The Home Office says it is aware of the concerns surrounding the side-effects of khat but that it stands by the decision in 2005 not to classify it after it took advice from the Advisory Council on the Misuse of Drugs.
It reported that "the extent of khat use in the UK is relatively low" and that the evidence of harm resulting from khat use is "insufficient, relative to those drugs that are under control".
However, it continues to keep an eye on the matter, with plans to improve understanding of khat misuse and the impact on communities.
But that doesn't go far enough for the Conservative Party, which plans to ban the plant if it gets into power.
Tory spokesman for social cohesion, Baroness Sayeeda Warsi, believes that advice and education about khat is simply not enough to tackle the problem.
"When there are barriers to the equality of opportunity and when there are barriers to integration - like a drug which is disproportionately affecting a certain community - then it is the responsibility of government to deal with that.
"This government's not dealing with it and that's why we're setting a very clear direction as to what we in government will do."