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Weeding out the truth about cannabis

Professor David Nutt makes a compelling case for the medical benefits of cannabis.

Renowned academic Professor David Nutt makes a compelling case for the medical benefits of cannabis. Book review by Mark Greener.

Until the early 1970s, UK clinicians could prescribe medicinal herbal cannabis. Today, access is strictly limited. Yet people in numerous countries, including the US, Holland, Belgium and Germany, can access medical marijuana, while several states across the US have legalised recreational cannabis. 

In Cannabis: Seeing Through the Smoke (Yellow Kite Books), David Nutt, Edmond J Safra chair in neuropsychopharmacology at Imperial College London, analyses the risks and benefits of cannabis. Professor Nutt chaired the Advisory Council on the Misuse of Drugs (ACMD) from 2000 until 2009, when he was sacked for publicly stating evidence-based views that conflicted with Government policy. 

Analyses by the ACMD showed, for example, that alcohol is more than twice as harmful as cannabis to users and five times more harmful than cannabis to other people.

“Alcohol is more harmful than cannabis in most ways in which harm can be measured. In particular, alcohol scored way higher on physical health, overdose risk and violence to others. There was only one harm where alcohol didn’t score higher, and that was dependence, where cannabis and alcohol were equal,” Professor Nutt comments in the book. 

Chemical composition

Delta-9 tetrahydrocannabinol (THC) is the main psychoactive component of cannabis but cannabis plants contain more than 400 chemicals. The plant’s chemical composition depends on the variety, preparation, growing conditions and harvesting,1 so medical cannabis is usually a standardised oil.

Cannabidiol accounts for up to 40 per cent of the plant extract and does not cause the euphoria induced by THC.2 The UK recently became the first country to regulate the market for cannabis extracts. Nevertheless, legal cannabidiol-containing oils, food and drinks are increasingly common on pharmacy and retail shelves (see the approved list). 

It is clear from Cannabis: Seeing Through the Smoke that cannabis and a group of active chemicals called terpenes offer marked benefits in certain diseases. Cannabidiol-based medicines are often effective in rare and treatment-resistant epilepsies, including Dravet and Lennox-Gastaut syndromes1, but whole plant extracts may be even more effective than pure cannabidiol.

“The best evidence so far for extracts of the whole plant is for childhood epilepsy. There is also a suggestion that extracts of the whole plant alleviate pain and anxiety,” Professor Nutt comments.

“The different terpenes can be titrated as well. For example, limonene can promote alertness and increased energy. Myrcene promotes sleep. We are also very excited by the growing use of medical cannabis for Tourette’s and Ehlers-Danlos Syndrome.” [This affects connective tissue so patients have hyper-flexible joints and stretchy, fragile skin.]

Of course, there is no pharmacological free lunch. Cannabis leaves some people feeling paranoid, confused and anxious. THC can also produce feelings of bodily distortions and visual hallucinations. The link between cannabis and psychosis, in particular, has caused considerable controversy.

People with psychiatric disease may ‘self-medicate’ with legal and illicit drugs, for example, but Professor Nutt says that studies have now shown a direct link. “Whole genome analysis shows that the genes for cannabis use are the same for risk overdose psychosis,” he says. 

The book notes that cannabis probably does not cause schizophrenia but may precipitate schizophrenic episodes in some genetically predisposed people. So pharmacists should warn those with a family history of psychosis to avoid cannabis, especially skunk, which is specifically grown to be high in THC. People with heart disease, hypertension or both should also be advised to be careful. Although rare, cannabis can trigger heart attacks.

“Alcohol is more harmful than cannabis in most ways in which harm can be measured”

Medically misleading

Regarding the risk of dependence, the book points out that “the usual statistic… that eight to 10 per cent of people who try cannabis become dependent” is medically misleading. 

“People who need insulin for diabetes are not addicts even though they are dependent on the medicine to stay well,” says Professor Nutt. “The same is true of people who are prescribed opiate painkillers for pain.” Moreover, most recreational users use skunk, which, because of the high THC levels, is more addictive than medical cannabis. 

“At the high doses used for epilepsy, cannabidiol can block some CYP450 enzymes,” Professor Nutt says. “THC can, obviously, leave you stoned, so pharmacists should remind people to not drive, operate heavy machinery and so on. THC can also aggravate the sedative effects of other drugs.” 

The legalisation of cannabis around the world offers a real-world experiment to assess cannabis’s risks and benefits – and it suggests that the risks have been exaggerated, says Professor Nutt. The benefits have not, however, been as well detailed, so Professor Nutt and colleagues have established Project Twenty21, which is gathering data about the efficacy of cannabis-based medicines for numerous conditions. “We now have more than 2,500 patients enrolled, who are using a range of medical cannabis products, from formulations of mostly cannabidiol to just THC and all ratios in between,” Professor Nutt says.

If positive, the results could bolster calls for the UK to bring legislation more in line with comparable countries. Apart from offering benefits for patients, legalising cannabis would transform drug crime and punishment. In 2020/21, possession of cannabis was the main drug offence, accounting for 63 per cent of cases in England and Wales. 

“The UK Government has persisted with prohibition policies, which … have not reduced the consumption of cannabis,” says Professor Nutt, and may exacerbate harms and fuel crime. Dealers sell a stronger version – skunk – and a dangerous synthetic substitute, spice, which can trigger extremely violent intoxication, seizures and heart attacks. Buying illegal cannabis could also raise the temptation to experiment with other more harmful drugs in the dealer’s bag.

The debate about the legal status of this ubiquitous herb is set to continue but Professor Nutt’s book throws considerable light on this contentious issue. “We need to move medical cannabis from Schedule 2 to Schedule 4 [of the Misuse of Drugs Act] and allow GPs to prescribe. Most of medical cannabis’s indications are dealt with in primary care,” Professor Nutt concludes.

“Medical cannabis is a fact and is likely to be an increasingly important aspect of future medicine.”


  1. Prescriber 2018; 29:12-16
  2. Surgical Neurology International 2018; 9:91-91
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