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Cannabis and cancer – is a new study cause for concern?

We break it down the facts behind a new study claiming to link cannabis to cancer.



Cannabis and cancer - is a new study cause for concern?
Experts say a new study linking cannabis to cancer should be approached with caution

Leading experts have questioned the legitimacy of a new study, which appears to link cannabis to cancer, saying medical consumers don’t need to be concerned. 

A recent study has claimed to show “strong” evidence that both THC and CBD – active ingredients in cannabis – are associated with the formation of multiple cancers.

Researchers at the University of Western Australia called the results “very concerning” and have concluded that use of cannabis should be urgently restricted worldwide.

But proponents of medical cannabis are sceptical of the report and have spoken out to Cannabis Health about their concerns as to the legitimacy of the research.

They have dubbed it “obscure science” with missing information which counters other studies and say it can be dismissed as “scaremongering”.

US research programme studies cannabinoids in ovarian cancer

The study

The findings of the study, which was conducted at the Division of Psychiatry, University of Western Australia, were published on 30 March.

The researchers used statistical software to access the census rates of 28 cancer types in all US states from 2001 to 2017 from the National Cancer Institute.They then accessed drug exposure information from the National Survey of Drug Use and Health 2003 to 2017.

Federal seizure data from the US Government provided details of cannabinoid exposure.

A total of 19,877 age-standardised cancer rates were returned. 

From the results they found that 15 cancers were more prevalent than usual in those who were exposed to high levels of CBD, the report said.

The researchers said the results were particularly concerning as more people across the globe are being exposed to cannabinoids.

It concluded that public access to cannabinoids should be carefully restricted “as a matter of public health and safety” and to retain the integrity of the food chain.

Doing this is a “non-negotiable” investment in the genomic health and prevention of cancers of multiple coming generations, the researchers stressed.

Discounting the report

Professor Mike Barnes, consultant neurologist and medical cannabis expert, told Cannabis Health that the report is “very obscure science” and makes little sense. 

“Frankly, I doubt this study and would be very cautious at accepting it at face value,” he said.

Most of the studies he is aware of have noted the anti-cancer properties of various cannabinoids and cannabis is often prescribed for cancer symptoms.

“This [report] is counter to that and implies cannabis can exacerbate cancer.”

Professor Barnes noted specifically that the data came from federal seizures, which makes it difficult to know exactly what the cannabis contains, such as contaminants like pesticides or mould.

The dose is unknown, as is whether the cannabis is synthetic or full spectrum.

“I think I would ignore this as it is potentially scaremongering for no good reason,” he added.

“All the evidence so far is that cannabinoids including CBD and THC are anti-cancer and not pro-cancer.”

Key limitations 

Dr Simon Erridge, head of research and access at Sapphire Medical Clinics, dug further into the report, concluding that he does not see any cause for concern.

“This is an interesting study which attempts to ask some interesting questions with respect to cancer risk related to exposure in the general population,” he told Cannabis Health.

“Whilst I commend the authors for their attempt to answer this question there are some key limitations to their analysis.”

The study is more representative of exposure to illicitly obtained cannabis through the black market where analysis of cannabinoid concentrations is from federally seized cannabis, rather than that sourced through dispensaries.

“As we are aware, illicitly obtained cannabis is more likely to be smoked, rather than administered via any other method,” he continued.

“We know that if you consume cannabis by smoking this causes the production of cancer-causing compounds in the same way as smoking tobacco and therefore it is not a recommended method of administration for medical cannabis.”

Administration of medical cannabis flower via vaporisation heats the cannabis flower to a lower temperature without igniting it.

This has been shown to significantly reduce the associated concentration of harmful chemicals produced, and is therefore the prescribed method of administration.

Dr Erridge’s major concern, he said, is that the study authors conclude that there are associations between some cancer types and cannabis, and specific concentrations of cannabinoids.

“However, the correlations as detailed in their analysis are weak and their analyses find almost as many incidences whereby higher cannabis and/or cannabinoid consumption is associated with a lower incidence of cancer, but they do not highlight this in this text,” he explained.

“The findings from their statistical tests therefore seem to be a result of having a high number of observations, leading to false positives with respect to both findings.”

Finally, there are large disparities between the types of cancer which are associated with cannabis and THC, he said.

Dr Erridge adds: “This is suspicious as one of the figures shows that THC is the cannabinoid which is prevalent in the highest proportions.”

Bizarre findings

Senior research fellow Dr Wai Liu of St George’s, University of London – who has been researching the anti-cancer properties of cannabis for most of his career – said the report is to be approached with some caution.

There is a lot of data contained within it, some of which seemed perplexing, he said.

For example, the study suggests that there is an inverse correlation between the consumption of alcohol and cancer of the liver ; which means the “more you drink, the less chance you have of getting liver cancer” – a finding he describes as “bizarre”.

“It’s the complete opposite of what I know to be the case from studies from the Cancer Research Institute, for instance,” he told Cannabis Health.

“All evidence shows a positive link between alcohol and cancer of the liver, but this paper is showing the complete opposite.

“We also don’t know what population they are looking at, so it’s such a mixed bag that it makes things very unclear.”

The paper also singles out CBD, but fails to state what amount of the cannabinoid was consumed.

“It’s all based on cannabis but they’re not sure what the percentages are of each cannabinoid, so it’s very messy and it’s not conclusive. I don’t think they can say CBD causes cancer because it’s CBD mixed with other things, such as THC and tobacco.”

Dr Liu has conducted several studies in cell and animal models and while not scientifically proven as a cure, he confirmed that cannabinoids “definitely have anti-cancer properties”.

He has heard hundreds of testimonies from patients who have benefited from using CBD while waiting for or alongside conventional cancer treatment, but he warns that papers such as this have led to patients stopping using CBD altogether.

“I’ve had patients bring this study to me and express concerns about its findings. Studies like this should come with a caveat making it clear that it isn’t looking at the cannabinoids individually but rather everything is lumped in together,” he added.

“Sometimes studies can be portrayed as clear and definitive – this is not the case here. I’d rather researchers say this and stress the limitations rather than present it as a complete scientific report.”

Other research 

According to Cancer Research UK, scientific research studies on the use of cannabis for cancer patients have had mixed results.

Some show that cannabinoids can cause cell death, block cell growth, stop the development of blood vessels needed for tumours to grow, reduce inflammation and reduce the ability of cancers to spread.

Meanwhile, scientists have also discovered that cannabinoids can sometimes encourage cancer cells to grow or cause damage to blood vessels.

“We need more research to know if cannabis or the chemicals in it can treat cancer,” the Cancer Research UK website reads.

“Clinical trials need to be done in large numbers where some patients have the drug and some don’t. Then you can compare how well the treatment works. 

“Many of the studies done so far have been small and in the laboratory. There have been a few studies involving people with cancer.”

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