High-strength skunk 'now dominates' UK cannabis market

Mental health
'Almost all cannabis seized by police now comprises high-strength varieties, with outdoor-grown herbal strains and hashish barely found' The Guardian reports

'Almost all cannabis seized by police now comprises high-strength varieties, with outdoor-grown herbal strains and hashish barely found' The Guardian reports

"Almost all cannabis seized by police now comprises high-strength varieties, with outdoor-grown herbal strains and hashish barely found," The Guardian reports. The news is based on researchers analysing samples of cannabis seized by 5 police forces in 2015 and 2016.

They found almost all the cannabis (93.6%) was sinsemilla, also known as skunk. This is a potent form of herbal cannabis often grown in the UK in indoor "cannabis farms" which has been specifically bred to have high levels of tetrahydrocannabinol (THC).

THC is the psychoactive chemical in cannabis responsible for many of the pleasurable effects people get when using the drug. THC has also been linked to an increased risk of developing mental health problems, such as psychosis (where a person is unable to tell the difference between reality and their imagination).

Analysis of samples of the drugs showed a wide variation in the levels of THC, with an average level of 14.2% in sinsemilla, compared to 6.3% in resin.

Cannabis resin, which dominated the UK market before 2000, only accounted for 5.8% of the cannabis seized by police. Resin THC levels were higher than those recorded in a similar study in 2005.

Cannabis resin usually contains cannabidiol (CBD), a substance thought to protect against some of the dangerous effects of THC. Sinsemilla does not usually contain CBD.

This matters, because doctors think people who use cannabis with high levels of THC, especially without CBD to counteract it, are more likely to become addicted and develop mental health problems.

Read more about the potential health risks of cannabis use.

Where did the story come from?

The study was carried out by researchers from GW Pharmaceuticals, which produces a range of medical cannabinoid medicines, and from King’s College London. GW Pharmaceuticals could be seen to have a conflict of interest in highlighting the dangers of illegal cannabis, as it is currently researching a range of medical treatments based on cannabidiol (CBD).

One of the researchers was funded by the Medical Research Council. The study was published in the peer-reviewed journal Drug Test Analysis.

The study was widely covered in the UK media. Some of the headlines – such as the Mail Online's "Terrifying rise of super-strength 'skunk' cannabis" - ramp up the fear, but by and large the study was accurately reported.

What kind of research was this?

This was a laboratory analysis of samples of drugs seized by police. The study gives a snapshot of the types of cannabis and range of potency of cannabis being sold illegally. However, we don't know how representative the samples are of the types of cannabis being used in the UK, as police may not target all potential cannabis users equally.

What did the research involve?

Researchers contacted 5 police forces which had been involved in previous surveys of cannabis, in 2005 and 2008. The forces were asked to send all seized cannabis samples in their property stores for analysis.

The researchers sorted the cannabis by type, then selected a representative sample to analyse for levels of THC, CBD, and a degraded form of cannabinoid, CBN, which is less potent and is a result of THC breaking down.

Cannabis came from Kent and London Metropolitan districts (in 2015) and Derbyshire, Merseyside and Sussex (in 2016). It was sorted into 3 types:

  • resin
  • sinsemilla
  • natural herbal cannabis (a less-potent type of cannabis, often imported from Morocco)

Researchers analysed about half of the samples of sinsemilla, and all of the samples of resin and natural herbal cannabis, as there were fewer of them. They analysed 250mg from each sample, which they say is a typical amount of cannabis used in 1 joint.

To see whether the time the sample had been held by the police affected the strength, they measured 34 samples where the length of storage was known, and looked at whether CBN levels were linked to time stored.

What were the basic results?

The vast majority of the 995 cannabis samples were sinsemilla:

  • 929 (93.6%) of these were sinsemilla, compared to 708 (84.5%) in 2008 and 247 (50.6%) in 2005
  • 58 (5.8%) were resin, compared to 104 (14.2)% in 2008 and 169 (42.7%) in 2005
  • 6 (0.6%) were traditional herbal cannabis, compared to 14 (1.3%) in 2008 and 39 (6.7%) in 2005

The average THC content of sinsemilla samples was 14.2%, similar to the 13.9% found in 2005. However, the range varied from 1.9% to 22.5%, with most being around 10% to 20%.

Average THC content of resin was much lower, at 6.3%, although this varied from no discernible THC to 29% in 1 sample found in a prison. The average strength was much higher than in 2005, when average THC concentration was 3.7%.

Only 1 of the sinsemilla samples contained CBD, the protective agent. While most resin samples did contain CBD, researchers found the average level had dropped from 4.3% in 2005 to 2.3% in 2015/6.

The analysis found no indication that length of time in police storage affected the strength of cannabis.

How did the researchers interpret the results?

The researchers said: "This trend presents an increased risk of harm to those susceptible to the development of psychotic disorders following cannabis use." They suggest the need for a nationwide survey.


Cannabis has often been dismissed as a relatively harmless street drug, compared to class A drugs such as cocaine and heroin. However, mounting evidence suggests it may have a harmful effect on mental health, particularly for teenagers and adolescents, increasing the chances of problems including panic attacks, anxiety and psychosis.

Although research is still taking place, it seems that stronger sinsemilla cannabis (aka skunk, which contains more THC and little CBD), raises the risk of mental health problems and addiction, compared to cannabis resin, which tends to have less THC and more CBD.

It's concerning that this study suggests sinsemilla is becoming much more common, and that where resin is on sale, it has more THC and less CBD than a decade ago. People who base their ideas about cannabis on the drug they smoked many years ago may not realise the strength and potential harm of the cannabis sold on the street today.

However, the study has some limitations:

  • It only looked at drugs seized by police. It's possible that police may prioritise arresting and confiscating drugs from people selling sinsemilla, because of its perceived harm. This might mean the proportion of cannabis resin in the study could be artificially low.
  • It only analysed a proportion of the samples of sinsemilla, and only 250mg from each sample. The overall sample strength might have varied, because the cannabis plant's concentration of THC varies in different parts.
  • Only 5 police forces were involved, so we don't know if the results would apply equally around the country.

Cannabis doesn't just affect mental health, smoking any form of cannabis can be bad for your health in other ways.

It can also:

  • damage your lungs
  • increase your risk of road accidents
  • damage your fertility and, if smoked when pregnant, damage the unborn baby

Find out more about the effects of cannabis.

Article Metadata Date Published: Wed, 28 Feb 2018
Author: Zana Technologies GmbH
NHS Choices