Kings College almost make a logical conclusion from a cannabis study, but fail to.



Another press release from Kings College London (KCL) about so-called “skunk” came out yesterday 27th February 2018:

UK cannabis market dominated by high-potency ‘skunk’

The first comprehensive survey of cannabis strength published in the UK for almost 10 years finds that high-potency varieties made up 94% of police seizures in 2016. The study, co-authored by King’s College London researchers, highlights the potential threat posed to mental health by a market dominated by strong cannabis.

Now this blog has criticised the work of Dr DiForti before (see here) as being based on very poor data collection and frankly that criticism still stands. However, if these claims are true, given this situation has arisen under the present drug policy and was arguably caused by it, you might expect KCL to be urging the government to take action to ensure a properly regulated commercial supply of higher CBD content cannabis. Surely that should be the urgent message to emerge from this study? Indeed, if there is any truth in the claims we hear from KCL about the risks from high potency cannabis and given it is now 10 years since they were first highlighted in the press, in no small part through the work of KCL, it would seem reasonable to expect them to demand action. After all, if true this is something that affects very large numbers – millions – of mostly young cannabis consumers who they claim to be concerned about.

Dr Di Forti, whose research into the mental health impacts of cannabis is funded by the Medical Research Council, says ‘More attention, effort and funding should be given to public education on the different types of street cannabis and their potential hazards. Public education is the most powerful tool to succeed in primary prevention, as the work done on tobacco use has proven.’

So no, they don’t call for any kind of reform, just more money aimed at “primary prevention”. As we will see, the study shows the alleged problem exists across the spectrum of street cannabis and it isn’t possible to make an informed choice about what types to use. Tobacco of course, isn’t a prohibited drug.

After 40 odd years of the Misuse of Drugs Act the only recommendation from these people is to call for more of the same failed tactics. This despite their research demonstrating that the problem they claim to have identified is getting worse. Of course there is an alternative, a regulated commercial supply where cannabis is provided at known and consistent potency levels.

For the record regulating the CBD content in commercial cannabis would be very easy to do under a legalised supply, it’s essentially simply a matter of growing the right strains and what used to be called “weights and measures” enforcement. Under a legalised supply consumers would at least know what was on offer – indeed we would control what was on offer – so consumers would be able to make an informed choice. Under the present regime, there is nothing that can be done. Zilch. More to the point we wouldn’t need studies like this because we would already know what was on sale and we could study the impact of the market with absolute certainty.

The data this all this is based on is worth looking at. The of full study can be read here and was undertaken by David Potter, Kathy Hammond, Shaun Tuffnell, Chistopher Walker and Marta DiForti. Apart from Dr DiForti declared conflicts of interest show:

David Potter, Kathy Hammond, and Shaun Tuffnell are employed by GW Pharmaceuticals group companies and are shareholders in GW Pharmaceuticals plc, a public company researching and developing a portfolio of cannabinoid medicines. Christopher Walker is employed by the Drug Control Centre, Kings College London.

This is worrying because most of these people would seem to be in a position to gain from the continued prohibition of cannabis, either commercially or through research grants which might explain their reluctance to promote law reform. The reason for saying that is a legalised commercial supply of cannabis would destroy GW Pharms business model and the need for dubious studies into the nature of street supplies. They cannot in all honesty be regarded as truly independent researchers because of this.

The study is worth looking at though, because it does show an important change. But first, a word about the data collection:

The materials had been seized from users and suppliers on the street or during searches of properties involved in cannabis dealing or consumption.

In other words, pretty much anywhere. So there was no methodology to the data sampling

A total of 995 seized cannabis samples were acquired from the same 5 constabulary areas (actually Derbyshire, Kent, London Metropolitan, Merseyside, and Sussex) included in the 2005 study. The differing forms were segregated, and a representative 460 samples analysed

The fact that the study was carried out in the same area in 2005 simply means the sample came from the same police source. it really means nothing much in terms of the objectivity of the statistical sampling. The differing forms were segregated by appearance, using a simple light microscope where necessary which is a debatable method in terms of being a really valid way to identify type of sample and adds an unknown potential for bias. The contents of seized herb grinders were also analysed.

However, if we treat this as a crude broad-brush indicative study it’s not too bad as they go and much better than most, it’s as good as we can expect under prohibition where data collection issues virtually guarantee cod science.

Many samples seized were found to be a mixture of cannabis and tobacco. All were excluded from this study. Approximately 1% of the seized samples, labeled by police as Green Vegetable Material, GVM, or similar, were not cannabis but were suggested to be so-called legal highs consisting of other plant species with added synthetic cannabinoid receptor agonists.

If that is anything like true and 1% of products sold a s cannabis being SCRA (“legal high fake cannabis”) this is especially worrying. Strange this hasn’t been picked up either by KCL or any of the press reports.

As to the results (click images for full sized version):

Range of sinsemilla THC contents in Derbyshire, Kent, London Metropolitan, Merseyside, and Sussex in 2004/2005 (n = 247) and 2015/2016 (n = 400)

Hash results are especially interesting

The varying THC:CBD:CBN ratios in illicit cannabis
analyzed in 2005 and 2016

In all but 1 case, the 400 sinsemilla samples were devoid of CBD, as were the few imported herbal samples, and they therefore lacked the pharmacologic qualities that this ingredient might otherwise offer. Due to its antipsychotic activity, CBD also has the potential to reduce the onset or degree of the psychoactive effects of THC.[23]

CBD was present in resin samples, but in widely varying ratios with THC.

So in addition to high potency herbal cannabis making up a higher percentage of street supplies, the CBD content of hash has been dropping in recent years. This shouldn’t come as a surprise given the increased availability of hash made form domestically cultivated cannabis and the drug-war efforts to choke off imports. There is nothing special about hash which would mean it naturally has a high CBD content. Hash will have the same profile as the plant it’s made from.

CBD is well known for reducing the psychotic effects of THC, but the claim that it reduces the psychoactive effects is dubious. From experience high CBD cannabis can be very psychoactive.

The potential effect on mental health is discussed

Although we reported that the THC content of sinsemilla has remained stable in England over the past 11 years, the decreasing availability of resin (the only reliable source of CBD) has reduced the overall proportion of CBD within the cannabis market. Moreover, our findings indicate that, in the few available resin samples, the average THC content has significantly increased, whereas average CBD content has decreased, compared to those of samples analyzed in 2005,[9] pointing at a cannabis street market dominated by high-potency varieties.

But of course, no discussion of the potential for a regulated supply, indeed instead the authors cherry pick a study from the USA which claims to show legalised cannabis causing an increase in psychotic harm. The inclusion of one cherry picked study looks suspiciously like a way to avoid discussion of law reform, an issue they must be well aware of.

Press coverage of all this has been muted, Perhaps because severe weather conditions and Brexit tensions have taken a higher profile today. The BBC carried the KCL press release without critical comment and no discussion on law reform, but with a section highlighting the legal penelties, pure prohibition supporting stuff. The Independent, while reporting the story, at least did a little digging: How hash and weed all but disappeared from Britain’s streets, as high-strength ‘skunk’ took over

It’s a mixture of clampdown by Moroccan government on resin trade so the UK resin supply sharply reduced”, he said. “Then into this gap in the market we have Vietnamese cannabis factory gangs coming to the UK, teaming up with UK domestic gangs, to grow and sell the skunk type weed.

“Also there has been a rise in online growing kits and advice. Now the Vietnamese have largely disappeared and cannabis farming is now big business for anyone from kids to organised crime groups. Also weed is still imported into this country mainly from Holland. There is now a lack of choice for consumers, and seems to be all about growing and smoking stronger weed, rather than the mellow hash of the 90s

So-called “reefer madness” claims are by no means proven, but if this problem of high potency cannabis leading to mental illness does exist as the authors imply, prohibition caused it and no solution is possible until prohibition ends. That must be obvious by now, so why no mention of the option in the study or KCL press release and little serious media coverage?


UKCIA is a cannabis law reform site dedicated to ending the prohibition of cannabis. As an illegal drug, cannabis is not a controlled substance - it varies greatly in strength and purity, it's sold by unaccountable people from unknown venues with no over sight by the authorities. There is no recourse to the law for users and the most vulnerable are therefore placed at the greatest risk. There can be no measures such as age limits on sales and no way to properly monitor or study the trade, let alone introduce proper regulation. Cannabis must be legalised, as an illegal substance it is very dangerous to the users and society at large.

3 thoughts on “Kings College almost make a logical conclusion from a cannabis study, but fail to.

  1. More great work from you Derek, thank you. It won’t surprise you to know that I reject your GW Pharma conspiracy theory.

    I own shares in GW Pharmaceuticals and I am president of the UK’s largest and longest established cannabis law reform group. the two are not mutually exclusive and I do not regard GW as the enemy. It is a small, innovative British company that with great courage, skill and ingenuity at enormous financial risk has defeated UK cannabis prohibition and become the greatest centre of expertise in cannabinoid medicine in the world.

    If GW does not have plans for entering the market with a massive, advantageous lead when legalisation finally arrives, I want to know why. To fail to do so would be massively irresponsible towards its investors and shareholders.

    This study is simply an analysis of cannabinoid content and GW’s scientists are probably better placed to do that than anyone else.

    You cannot lay the blame on GW for distorting the results of this study into scaremongering about the tiny risk of psychosis. That’s entirely down to Marta di Forti, Robin Murray, etc. Every year at about this time they release their latest ‘skunk’ scare story. All the media hype is what helps to raise their funding for the next year’s scaremongering – and so we go on.

  2. Peter I simply make the point, which is factual, that the legalisation of herbal cannabis for medicinal use, which clearly applies if non-medical use is legitimised, will undermine the business case for GW Pharms and that shareholders and employees of GW will suffer a loss as a result. It is therefore reasonable to point out that these people have a financial incentive to oppose cannabis law reform. I am not saying they are doing this, only that the incentive exists.

    However, that said it is worth noting the final couple of paragraphs in section 3.9 which cherry picks a study in a thinly disguised warning against law reform, viz:

    “Finally, data from the United States on the impact of widespread changes to cannabis legislation are becoming available. An article recently published by Hasin et al. indicated that, in US states such has California that have passed medical marijuana laws, both an increased prevalence of illicit cannabis use and, more importantly, of cannabis use disorders has been noted. The authors concluded:

    “While medical marijuana may help some, cannabis-related health consequences associated with changes in state marijuana laws should receive consideration by health care professionals and the public.” ”


    They did this rather than discuss the potential of regulation to address the problem they cliam to have identified.

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