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What is cannabis?

Cannabis is a Class B drug derived from the cannabis plant, a bushy plant found wild in most parts of the world and easily cultivated in Britain. There are three varieties of the plant, Cannabis sativa, indica and ruderalis. In Western countries it is generally used as a relaxant and mild intoxicant.

The most important psychoactive ingredients are the tetrahydro-cannabinols (THC).The availability and supply of cannabis has profoundly changed since the early 2000’s, with the previously widespread imported resin largely replaced by home grown herbal variants of the drug. Cannabis farms often located in suburban homes or commercial warehouses in major urban areas now supply the vast majority of the UK market. In the last couple of years, synthetic variants such as Spice have also made some headway into the market. Significantly less common in the UK is cannabis oil, generally prepared by percolating a solvent through the resin.

Though still the most prevalent illegal drug in the UK, statistics suggest a long term downward trend for cannabis use. There are suggestions that the drug can in rare cases trigger psychosis, a factor that led to the government in 2009 to reclassify cannabis back to Class B after previously downgrading it to Class C in 2004.

Cannabis photo

Cannabis © Thorsten Ulbricht - FOTOLIA

Legal status

Having been briefly reclassified to Class C from 2004, cannabis in its various forms was reinstated as a Class B drug in 2009 under regulations that prohibit its medical as well as its non-medical use. This means it is illegal to cultivate, produce, supply or possess the drug, except in accordance with a Home Office licence issued for research or other special purposes. It is an offence to allow premises to be used for producing (including cultivating), supplying of or smoking cannabis. This last type of offence – allowing the use of a drug – applies only to permitting the smoking of cannabis or opium. More on the law, including penalties, regarding cannabis can be found here.

There are some exemptions under the Misuse of Drugs Act in relation to research and development and prescription of cannabis-based medication.In 1995 the Misuse of Drugs Act was amended, placing dronabinol (a THC derivative), into Schedule 2, allowing doctors to prescribe the drug to named cancer patients for use as an anti-emetic. Sativex, a pharmaceutical company, has already received a licence from the government to grow low-THC content cannabis to help treat patients with multiple sclerosis (MS).


Many people believe that cannabis is becoming stronger. The EMCDDA released an European review of cannabis potency in June 2004. The study revealed that when the overall potency of cannabis products on the market is calculated, there is no evidence of a significant increase in potency. This is because, in most EU countries, imported cannabis dominates the market and this has remained stable over many years. A report of the study can be found on EMCDDA Drugnet (July-Sept 2004)

The full report can be found here.

A more recent cannabis potency study, published by the Home Office in Spring 2008, can be found here.

Production and supply

While the UK has a long history of importing cannabis resin, particularly from North Africa, the home grown market expanded significantly over the course of the 2000’s. A 2010 report from the Association of Chief Police Officers found that herbal cannabis cultivated in farms, often located in rented properties and commercial units, now makes up around 70 to 80 percent of the commercial supply.[i] As recently as 2002, 75 to 80 percent of cannabis was imported from Morocco via Spain and Gibraltar.[ii]

The last few years has seen the number of farms closed down by the police more than double. Between 2004 and 2007, police detected around 800 cannabis farms per year in the UK. This had risen to 7,000 by 2009/10, with the largest concentrations located in West Yorkshire, Greater Manchester and the West Midlands.[iii] A total of 750,000 cannabis plants were recovered by the police in 2009/10.[iv]


Due to the changed nature of the market, with the majority of the UK supply now home grown herbal cannabis produced indoors without soil under powerful lights, the average THC content has risen from around 7-8% to 12-14% today. However, claims that these newer cannabis strains, often referred to as skunk, are 25 times stronger now than they were a few decades ago are largely unfounded.[v] A 2008 study commissioned by the Home Office found that the mean THC concentration in England and Wales was 8.4% for herbal cannabis, and 5.9% for cannabis resin.[vi]

It is believed that commercial growers have been harvesting cannabis plants early in order to turn a quick profit which decreases the potency of the product.

In 2007, the Department of Health issued three warnings about samples of herbal cannabis that were contaminated with minute particles of glass, which became known among users as ‘gritweed’.Some cannabis growers were alleged to have sprayed the cannabis with the glass particles in order to make it appear ‘sticky’, indicating a higher grade drug.This was a rare instance of drug contamination with a potentially harmful adulterant. It is thought that ‘gritweed’ is no longer sold on the UK market.


Cannabis has the greatest non-medical usage of all the drugs controlled under the Misuse of Drugs Act. Overall, about 10 million people in the UK would admit to having tried it, with around 1.25 million reporting using it in the last month. Over a third of 16-24 year olds, or around 2.3 million people, have taken it at least once in their lifetimes.[vii]

According to the British Crime Survey, it is estimated that around 1 million 16-24 year olds, or 16.1% of that age group cohort, used the drug in 2009/10. However, trends suggest that among the 16-24 and 16-59 age groups, cannabis use has been falling for some time.[viii]

Licit and illicit use

In the UK, cannabis is generally smoked with tobacco in a joint or spliff, but can also be smoked in a pipe, brewed into a drink or cooked into food. Regular users might consume one-eighth of an ounce per week; heavy and regular cannabis users might use that amount in a day. Roughly one-sixteenth ounce of cannabis resin would be sufficient to produce four strong or eight less potent joints. Joints are typically smoked with friends at any one time, although many also enjoy using the drug alone to enhance their perception of music or simply to relax.

There is growing evidence that cannabis may be effective in providing symptomatic relief for diseases of the muscular-skeletal system like multiple sclerosis, as an anti-nausea drug in chemotherapy, and to relieve intra-ocular pressure in glaucoma patients. In recent years certain parts of the United States, including California, have issued licenses for medical consumption of the drug.


Average prices per quarter ounce of standard quality herbal resin were £30 in 2010. For good quality herbal cannabis the price rose to £50, while for resin cannabis the average price was £21. There has been some indication in recent years that street dealers have moved to metric measurements to weigh sales. [ix]


Smoking cannabis causes a number of physical effects including increased pulse rate, decreased blood pressure, bloodshot eyes, increased appetite and occasional dizziness. These can start within a few minutes and may last several hours depending on how much is taken. When eaten the effects take longer to start but may last longer. Eating cannabis may mean a large dose is taken in one go, making it difficult to avoid any unpleasant reactions.

While use often relies on the expectations, motivations and mood of the user, as well as the amount taken and the surroundings the drug is taken in, cannabis can lead to a state of relaxation, talkativeness, the giggles, and greater appreciation of sensory expectations. It is regularly taken to enhance or detract from the effects of other drugs such as ecstasy or cocaine, particularly after long dance sessions.

While intoxicated, cannabis can affect the short-term memory, concentration, and intellectual or manual dexterity, including driving. Higher doses can lead to perceptual distortion, forgetfulness and confusion of thought processes. Temporary psychological distress and confusion can occur, particularly among inexperienced users or if the user is feeling anxious or depressed.

The physical effects of inhaling cannabis can impact on the respiratory system, leading to oral, throat, and lung cancer. Psychologically, use of cannabis has been reported to cause anxiety and paranoia in some users and may in rarer cases be a trigger for underlying mental health problems. The Advisory Council on the Misuse of Drugs found a ‘probable but weak causal link between psychotic illness and cannabis use.’[x] While cannabis does not produce physical dependence, mild withdrawal symptoms have been produced in experiments. Skunk has more psychoactive properties than resin as it contains higher levels of the active chemicals, and early use has been associated with later mental health problems.

A gateway drug?

It is often suggested the taking of one drug, particularly cannabis, may lead to the use of other drugs. While it is true that most people who use heroin will have previously used cannabis, they are also likely to have smoked tobacco and consumed alcohol. Only a small proportion of those who try cannabis go on to use heroin. Importantly, though, cannabis use involves people in the buying of illegal drugs, making it more likely that they will meet with an offer of heroin, an offer which some will accept. In this example, it would be the illegality of cannabis use rather than cannabis use itself that leads most directly to contact with heroin. The Dutch Ministry of Justice, for example, has suggested that a process of escalation does exist, but only where a single criminalised market is in place. The escalation or gateway theory often features in the debate on whether cannabis should remain illegal or not. With the threat that its use may lead to the use of other drugs, many take the stance that it should remain illegal.

Updated 2011

[i] Findings from the UK National Problem Profile Commercial Cultivation of Cannabis ACPO 2010

[ii] The Essential Guide to Drugs and Alcohol, 14th Edition, Shapiro, 2010

[iii] Findings from the UK National Problem Profile Commercial Cultivation of Cannabis ACPO 2010

[iv] The Essential Guide to Drugs and Alcohol, 14th Edition, Shapiro, 2010

[v] Cannabis: An Apology, The Independent on Sunday, 18 March 2007

[vi] Home Office Cannabis Potency Study, Hardwick and King, 2008

[vii] British Crime Survey 2010

[viii] British Crime Survey 2010

[ix] Druglink magazine Volume 25 Issue 5 2010

[x] The Essential Guide to Drugs and Alcohol, 14th Edition, Shapiro, 2010