| Cannabis: scientific background |
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What is cannabis? Cannabis is a plant that is smoked or sometimes eaten (e.g. hash brownies) for its psychological effects. Cannabis resin is sometimes called ‘hash’ and the flowers and leaves are sometime called ‘weed’. Selectively bred strains of high-potency cannabis are referred to as ‘skunk’. Cannabis contains a number of chemicals called ‘cannabinoids’ and the principal psychoactive cannabinoid is ‘delta-9-tetrahydrocannabinol’ (THC). The use of cannabis as a drug dates back to ancient China where the cannabis plant is indigenous. Nomadic tribes are thought to have brought cannabis west to Western Asia and Arabia. Western Europe became aware of the psychoactive properties of cannabis after the Napoleonic invasion of Egypt. In the alcohol-free state, the consumption of cannabis-based products was prevalent. French soldiers developed a liking for the drug and in the mid-nineteenth century cannabis became popular among French intellectuals who subsequently wrote about their experiences (e.g. the psychiatrist Jacques-Joseph Moreau, 1845). After hundreds of years of cultivating cannabis-based products for food and fibre, recreational cannabis use only became significantly prevalent in Britain in the 1960s, although it had been illegal for decades. Cannabis is now the most widely used illegal drug in the UK. Cannabis use is especially prevalent among young people. It is difficult to estimate its prevalence in the UK but approximately 50% of people aged 30 and under are likely to have taken cannabis in their lifetime (ACMD, 2008). Many countries recognise its medicinal value in their legislature. Why do people take cannabis? By far the most often given reason for taking cannabis is to relax and relieve stress. Psychological insight/personal development and mood improvement are also given by a small number of users. It is also likely that some young people take cannabis for social and thrill-seeking reasons. Some people take cannabis to help with physical pain and suffering. The analgesic (pain-relieving) and antiemetic (nausea-relieving) effects of cannabis have been recognised for centuries and some countries now permit the use of cannabis for these medicinal purposes. Although some users report that cannabis helps with anxiety and depression, there is little evidence to support this. It may be that the acute effects of cannabis provide temporary stress relief for some people, which is maintained through regular use. Some users appear to be able to tolerate the potential negative psychological effects of cannabis (e.g., apathy, social withdrawal and paranoid ideation) better than others and more research is needed to understand why this is the case. How does cannabis work in the brain? The major psychoactive effects of cannabis are mediated by THC (Mechoulam, 1970). THC stimulates cannabinoid receptors in the brain. The brain contains endogenous (naturally occurring) cannabinoids (endocannabinoids) and THC acts at the receptor site meant for these chemicals (the cannabinoid 1 receptor (CB1)). The psychoactive effects of cannabis are mediated by THC’s action at the CB1 receptor. CB1 receptors are expressed throughout the brain on the terminal parts of neurons - where neurotransmitters are released. Stimulation of CB1 receptors has been shown to inhibit neurotransmitter release. Endocannabinoids are released ‘on demand’ in the brain and function to inhibit neurotransmitter release via stimulating the CB1 receptor. In this way they regulate mood and cognition (thought) in a flexible, context-sensitive manner. THC may act to disrupt this regulatory function and this may account for cannabis’s psychoactive properties (see Iversen, 2008 for a review). What are the potential harms of cannabis? Most people consume cannabis by smoking it and most include tobacco in the mixture they smoke. The harms of tobacco smoke are well recognised but the harms of cannabis smoke are not. Analyses of the combustion products of pure tobacco and cannabis have shown them to be similar and cannabis smoke is inhaled more deeply than tobacco smoke (Iversen, 2008). The potential harms of smoking cannabis are sometimes argued to be less than the harms of smoking tobacco cigarettes because the latter are smoked more regularly and for a longer period of a person’s life. However, cannabis joints are often mixed with significant amounts of tobacco and some regular users smoke tobacco cigarettes in the periods between smoking cannabis joints. The smoking of any tobacco-containing product can lead to dependence due to the addictive properties of nicotine. It has long been recognised that cannabis (or more accurately THC) can cause temporary psychotic symptoms (Moreau, 1845; Weil, 1970). The risk of this is increased if the cannabis contains a high proportion of THC in relation to another ingredient of cannabis, cannabidiol, which may counteract the psychoactive effects of THC (Morgan & Curran, 2008; Bhattacharyya et al. 2010). The THC content of cannabis has increased dramatically in recent years. Although it is well established that cannabis can cause transient psychotic symptoms in normally healthy individuals and negatively affect treatment outcome in patients with schizophrenia (Zammit et al. 2008) the idea that it can cause schizophrenia in healthy individuals is controversial (Nutt, 2009). What seems more likely is that cannabis can promote psychotic illness in individuals that are already vulnerable to this. Cannabis has a very low-level of toxicity and presents a relatively low risk of dependency (ACMD, 2008). For these reasons, it is typically considered a relatively safe drug (Nutt et al. 2007). This is not to say that cannabis is harmless, only that its harms are relatively less serious than those associated with other drugs. Studies have associated long-term cannabis use with poor educational achievement and psychological health (e.g., Macleod et al. 2004) but it is difficult to determine whether cannabis is a cause or effect of this. In a survey of 620 cannabis users, 43% reported that cannabis had probably (30%) or definitely (12%) caused or made worse a physical or mental health problem but users also rated cannabis as having the least serious negative effects (Carhart-Harris & Nutt, 2008). The most prevalent negative effect of cannabis reported by users is apathy (see Iversen, 2008). What is the current legal status of cannabis? Cannabis was classified in most countries in the late 1920s. In 1971, under the Misuse of Drugs Act, cannabis was made Class B. In 2002, The Advisory Council on the Misuse of Drugs (ACMD) recommended that cannabis be reclassified to Class C based on an assessment of its relative harms. This was implemented in 2004. Based on fresh fears about the increasing potency of cannabis and associated mental health risks, at the request of the government, the ACMD carried out two reviews of the evidence on cannabis, in 2005 and 2008, advising on both occasions that cannabis remain Class C. Cannabis was reclassified to Class B in January 2009. Possession of Class B drugs is punishable by up to 5 years imprisonment and dealing or importation can result in 14 years imprisonment. References Advisory Council on the Misuse of Drugs (2008) Cannabis; classification and public health. http://drugs.homeoffice.gov.uk/publication-search/ acmd/acmd-cannabis-report-2008? 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