Media playback is unsupported on your device Media caption Former skunk user Liaquat Zaman: "It brought me to a very dark place in my life" 
 
 Smoking potent cannabis was linked to 24% of new psychosis cases analysed in a study by King's College London. 
 
 The research suggests the risk of psychosis is three times higher for users of potent "skunk-like" cannabis than for non-users. 
 
 The study of 780 people was carried out by KCL's Institute of Psychiatry, Psychology and Neuroscience. 
 
 A Home Office spokesman said the report underlines the reasons why cannabis is illegal. 
 
 Scientists found the risk of psychosis was five times higher for those who use it every day compared with non-users. 
 
 They also concluded the use of hash, a milder form of the drug, was not associated with increased risk of psychosis. 
 
 Psychosis refers to delusions or hallucinations that can be present in certain psychiatric conditions such as schizophrenia and bipolar disorder. 
 
 Risk increased 'threefold' 
 
 "Compared with those who had never tried cannabis, users of high potency skunk-like cannabis had a threefold increase in risk of psychosis,' said Dr Marta Di Forti, lead author on the research. 
 
 She added: "The results show that psychosis risk in cannabis users depends on both the frequency of use and cannabis potency." 
 
 Dr Di Forti told BBC Radio 4's Today programme that the availability of skunk-like cannabis was becoming more widespread. 
 
 Image copyright AP 
 
 'I was a wreck' 
 
 Cath from Berkshire, who asked to remain anonymous, believes smoking skunk caused her to experience mental health problems. 
 
 "I dabbled with a friends' group in my early 20s, and went from someone who had never experienced any mental health issues whatsoever, to an absolute wreck. 
 
 "I was terrified of leaving the house, and I became petrified of death, of the mysteries of the universe, and of being alone. You name it, I was terrified of it. 
 
 "It took about six years to feel normal again and now, almost 20 years later, I have absolutely no doubt that my issues were triggered by casually and naively smoking this so called 'soft' drug. 
 
 "For years I have shuddered as campaigners have sought to declassify or promote the product as I understand first-hand the hidden yet, until now, unspoken dangers of this awful drug." 
 
 "In London, it's very difficult to find anything else," Dr Di Forti said. 
 
 "There were lots of reports from police across the UK saying we have become a great producer of skunk. And not only do we use it locally but we export, so this is a Made in England product." 
 
 Someone suffering from psychosis would often be "extremely paranoid and become very suspicious" about the people around them, she added. 
 
 She has called for "a clear public message" to cannabis users, comparable to medical advice on alcohol and tobacco. 
 
 GPs should be encouraged to ask how often and what type of cannabis patients use, she added, 
 
 A Home Office spokesman said the findings backed up the government's approach: "Drugs such as cannabis are illegal because scientific and medical evidence demonstrates they are harmful. 
 
 "This report serves to emphasise how they can destroy lives and communities." 
 
 'Without risk' 
 
 Skunk contains more THC - the main psychoactive ingredient - than other types of cannabis. 
 
 Unlike skunk, hashish - cannabis resin - contains substantial quantities of another chemical called cannabidiol or CBD and research suggests this can act as an antidote to the THC, counteracting psychotic side effects. 
 
 Sir Robin Murray, professor of psychiatric research at King's, commented: "This paper suggests that we could prevent almost one quarter of cases of psychosis if no-one smoked high potency cannabis. 
 
 "This could save young patients a lot of suffering and the NHS a lot of money." 
 
 The research was carried out over several years, comparing 410 patients aged 18-65 who reported a first episode of psychosis at a south London psychiatric hospital with 370 healthy participants within the same age range from the same area of London. 
 
 It will be published later this week in the Lancet Psychiatry. 
 
 Rosanna O'Connor, director of alcohol, drugs and tobacco at Public Health England, responded: "No drug use is without risk as this report demonstrates. 
 
 "Anyone having problems with drug use should seek help from their local specialist drug services. It is important to remember that treatment for all types of drug problems, including cannabis, are readily available and very effective". 
 
 Cannabis user Robert, from Hertfordshire experienced a "temporary psychosis" after taking home-grown cannabis in his 20s. 
 
 "It was utterly terrifying, and the worst night of my life," he told the BBC News website. 
 
 "As someone affected by this issue it is hard watching mainstream media, particularly comedy films, portraying cannabis as a harmless life-enhancing substance with limited ill-effects - it's simply not true." 
 
 Another person who contacted the BBC website felt that legalising cannabis would allow varieties to be regulated. 
 
 Phil, from Cambridgeshire, said: "Speaking as a 'toker' for past 25 years, super skunk is the term they should be using. 
 
 "The cause of all this is the illegality. If things were more open and informed, people could buy 'saner' variety seeds." ||||| One in four cases of psychosis in Britain is associated with powerful strains of cannabis such as "skunk", a study has found. 
 
 Scientists found such potent forms of the drug increased the likelihood that a user would develop the condition. 
 
 Casual users of the stronger variants of the class B substance have been found to be three times more likely to develop psychosis, while heavy users are five times more likely. 
 
 The findings of the six-year study - which have been published in the Lancet Psychiatry journal - suggest strong forms of cannabis can lead to serious mental illness. 
 
 Sarah Graham, a drugs counsellor, has claimed the figure is "just the tip of the iceberg". 
 
 Play video "2014: Inside A UK Cannabis Factory" Video: 2014: Inside A UK Cannabis Factory 
 
 A recovering addict herself, she described "skunk" as the "Incredible Hulk of cannabis", and warned: "It is absolutely decimating young people's lives." 
 
 She told Sky News: "It's not the weed that many parents may have experimented with at university or college. This is a very, very potent drug. It’s like a turbo-charged version of ordinary cannabis - and it's laced with chemicals, too. 
 
 "I see so many young people who are having their mental health decimated by this drug. I see them in hospitals, in psychiatric wards and it is absolutely heart-breaking. I see them being diagnosed with forms of schizophrenia. We need to start taking this seriously." 
 
 Researchers from King's College London say there should be greater public awareness of the effects of using the stronger form of the drug. 
 
 Sir Robin Murray, professor of psychiatric research at the university, said: "It is now well known that use of cannabis increases the risk of psychosis. 
 
 "However, sceptics still claim that this is not an important cause of schizophrenia-like psychosis. 
 
 "This paper suggests that we could prevent almost one quarter of cases of psychosis if no one smoked high-potency cannabis. 
 
 "This could save young patients a lot of suffering and the NHS a lot of money." 
 
 While skunk was associated with psychotic episodes, the milder form of cannabis known as hash had no such link, the scientists said. 
 
 The study looked at 800 people aged between 18 and 65 in south London, including 410 who had suffered psychosis and 370 healthy patients. ||||| The study by King's College London found that smoking "skunk-like" cannabis triples the risk of psychosis. The drug was linked to 24 per cent of new psychosis cases, according to the research. 
 
 The researchers say there is an "urgent need" to inform young people about the risks of strong cannabis. But what about alcohol? 
 
 Sir Robin Murray, professor of psychiatric research at King's College London, told Channel 4 News: "Most people who drink alcohol do it moderately and enjoy it. Most people who smoke cannabis do it moderately and enjoy it. 
 
 "But if you drink a bottle of vodka each day it won't be good for your health, the same with a large amount of cannabis." 
 
 Physical damage 
 
 
 
 Professor Murray said that in terms of physical damage, alcohol was more dangerous. 
 
 "In absolute terms, far more people die of alcohol than cannabis. It's very difficult to kill yourself from cannabis. It may make you do some crazy things, but you're not going to get kidney or liver disease from cannabis. 
 
 "In terms of the psychological problems it's different. If you drink heavily you'll get dementia eventually. But with cannabis, there's still a dispute." 
 
 Professor Murray said there were relatively few studies on the psychological effects of cannabis, and there was "still an argument" about whether the cognitive impairment of a cannabis user returns to normal after heavy use. 
 
 Psychological damage 
 
 
 
 "In terms of hallucinations that is much more common with cannabis. If you're lucky you'll recover. But if you smoke cannabis for many years you may develop schizophrenia." 
 
 Comparing the damage to non-users and society is difficult, said Professor Murray, mainly because one drug is legal and the other is not. 
 
 "At present, alcohol does much more damage than cannabis. But of course a huge proportion of the male population is drinking every day - sometimes heavily. Nothing like the same proportion are smoking cannabis every day." 
 
 In 2010, a study published in the Lancet on behalf of the independent scientific committee on drugs found that heroin, crack cocaine and methamphetamine (crystal meth) were the most harmful drugs to individuals. 
 
 The most harmful drugs to those in contact with users were alcohol, crack cocaine and heroin. ||||| Study suggests high levels of skunk use may affect the brain’s white matter, making communication between the right and left hemispheres less efficient 
 
 High-strength cannabis may damage nerve fibres that handle the flow of messages across the two halves of the brain, scientists claim. Brain scans of people who regularly smoked strong skunk-like cannabis revealed subtle differences in the white matter that connects the left and right hemispheres and carries signals from one side of the brain to the other. 
 
 
 
 The changes were not seen in those who never used cannabis or smoked only the less potent forms of the drug, the researchers found. 
 
 
 
 Cannabis: what's in a name? Read more 
 
 The study is thought to be the first to look at the effects of cannabis potency on brain structure, and suggests that greater use of skunk may cause more damage to the corpus callosum, making communications across the brain’s hemispheres less efficient. 
 
 Paola Dazzan, a neurobiologist at the Institute of Psychiatry at King’s College London, said the effects appeared to be linked to the level of active ingredient, tetrahydrocannabinol (THC), in cannabis. While traditional forms of cannabis contain 2 to 4 % THC, the more potent varieties (of which there are about 100), can contain 10 to 14% THC, according to the DrugScope charity. 
 
 
 
 “If you look at the corpus callosum, what we’re seeing is a significant difference in the white matter between those who use high potency cannabis and those who never use the drug, or use the low-potency drug,” said Dazzan. The corpus callosum is rich in cannabinoid receptors, on which the THC chemical acts. 
 
 
 
 Facebook Twitter Pinterest A DTI image of the corpus callosum, as seen from the side, is shown in red on and superimposed on a background MRI image of the brain. Photograph: Institute of Psychiatry 
 
 “The difference is there whether you have psychosis or not, and we think this is strictly related to the potency of the cannabis,” she added. Details of the study are reported in the journal Psychological Medicine. 
 
 
 
 The researchers used two scanning techniques, magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI), to examine the corpus callosum, the largest region of white matter, in the brains of 56 patients who had reported a first episode of psychosis, and 43 healthy volunteers from the local community. 
 
 
 
 The scans found that daily users of high-potency cannabis had a slightly greater – by about 2% – “mean diffusivity” in the corpus callosum. “That reflects a problem in the white matter that ultimately makes it less efficient,” Dazzan told the Guardian. “We don’t know exactly what it means for the person, but it suggests there is less efficient transfer of information.” 
 
 
 
 The study cannot confirm that high levels of THC in cannabis cause changes to white matter. As Dazzan notes, it is may be that people with damaged white matter are more likely to smoke skunk in the first place. 
 
 “It is possible that these people already have a different brain and they are more likely to use cannabis. But what we can say is if it’s high potency, and if you smoke frequently, your brain is different from the brain of someone who smokes normal cannabis, and from someone who doesn’t smoke cannabis at all,” she said. 
 
 So smoking skunk causes psychosis, but milder cannabis doesn't? Read more 
 
 But even with the uncertainty over cause and effect, she urged users and public health workers to change how they think about cannabis use. “When it comes to alcohol, we are used to thinking about how much people drink, and whether they are drinking wine, beer, or whisky. We should think of cannabis in a similar way, in terms of THC and the different contents cannabis can have, and potentially the effects on health will be different,” she said. 
 
 “As we have suggested previously, when assessing cannabis use, it is extremely important to gather information on how often and what type of cannabis is being used. These details can help quantify the risk of mental health problems and increase awareness of the type of damage these substances can do to the brain,” she added. 
 
 In February, Dazzan and others at the Institute of Psychiatry reported that the ready availability of skunk in south London might be behind a rise in the proportion of new cases of psychosis being attributed to cannabis.
– Despite efforts to reclassify marijuana down to a Schedule II (read: less dangerous) drug, there are some people who still insist on its dangers—and new research may back those theories up. Scientists at King's College London have discovered that smoking potent, or "skunklike," cannabis was tied to almost a quarter of all new psychosis cases they studied, the BBC reports. The study published in the Lancet compared 410 patients between the ages of 18 and 65 who had experienced "first-episode psychosis" with 370 healthy patients. The results suggest that the development of psychosis—i.e., delusions and hallucinations—is three times greater for skunk smokers than for nonusers, and up to five times higher if they smoked it every day. Hashish, a milder form of cannabis that contains much less THC than the skunk variety, isn't linked to any increased risk for psychosis. English experts say that skunk cannabis is widespread, with one doctor telling the BBC, "In London, it's very difficult to find anything else." Anecdotal stories that back up the study's warnings are worrisome. One woman tells the BBC that, after smoking skunk pot, she "went from someone who had never experienced any mental health issues whatsoever to an absolute wreck." A recovering addict who's now a drug counselor tells Sky News that skunk is the "Incredible Hulk of cannabis [and is] absolutely decimating young people's lives." Still, despite the warnings, study researchers point out that, according to a 2010 study in the Lancet, the three most harmful drugs are alcohol, crack cocaine, and heroin; cannabis falls at No. 8, notes Channel 4 News. (It's not the first time suspicions have been raised linking skunk cannabis to psychosis.)