"Smoking cannabis can alter a person's DNA, causing mutations that expose a user to serious illnesses," the Mail Online reports.
A new review has looked at the role cannabis may play in what is known as chromothripsis.
A relatively recent discovery, chromothripsis is when the DNA of a cell suffers large-scale damage, but not enough to kill the cell. It has been linked to some types of cancer and birth defects.
In this review, researchers considered the evidence about whether one of the active ingredients in cannabis – tetrahydrocannabinol (THC) – could trigger chromothripsis, which could potentially cause cancer and other illnesses.
The researchers also raised the possibility that the DNA damage could be passed down to later generations.
There is a great deal of uncertainty about how the included studies were chosen, so there is a possibility not all relevant research was considered.
This type of study serves to stimulate debate and further research. It is not reliable enough to form the foundation of policy change on its own.
Arguably, a longer-term study would be needed to see if cannabis use could have an intergenerational effect.
We do know that cannabis, a class B illegal drug, is known to contain cancer-causing chemicals (carcinogens) and has previously been linked with lung cancer, psychosis, schizophrenia and fertility problems.
Find out more facts about cannabis.
Where did the story come from?
The review was carried out by two researchers from the University of Western Australia. There was no external source of funding.
It was published in the peer reviewed journal, Mutation Research: Fundamental and Molecular Mechanisms of Mutagenesis.
The Mail Online's headline, "Smoking cannabis can alter a person's DNA, causing mutations that expose a user to serious illnesses", made it sound like the researchers' hypothesis was proven by newly uncovered evidence, which is not the case.
The headline and article did largely reflect the researchers' findings, but failed to add any notes of caution, balance or discussion about the limitations of the research, instead taking it at face value.
What kind of research was this?
This was an evidence-informed narrative review of research exploring the hypothesis that cannabis use causes errors in human DNA, potentially leading to cancer and affecting brain development in unborn babies.
Non-systematic reviews like this are useful for summarising scientific research in a particular area, but can miss relevant research and counter-arguments.
Without a clear and systematic review of the published and unpublished science, there is a risk the authors cherry-picked the evidence, consciously or unconsciously, to fit their views.
Such a one sided-argument has its place in stimulating debate, but should not be viewed on a par with a systematic review, one of the highest levels of evidence.
A systematic review of well-designed long-term cohort studies would be one of the best ways to assess the causal links between cannabis and DNA damage and disease.
What did the research involve?
The research is a narrative review of evidence that presents the idea that cannabis can disrupt a person's DNA, potentially raising their risk of cancer and causing genetic toxicity that could be passed from one generation to the next.
The review assembled data from 189 research articles. However, it had no reported methods. As such, we cannot assume the researchers employed systematic review methodology.
As the authors didn't mention how they found the articles, the study risks being biased to fit a coherent story, or may have missed other relevant research.
Some limitations in the evidence were presented, although quite briefly. The relative strength and balance of evidence for and against their hypothesis is not clear.
What were the basic results?
The review starts by providing scientific background on key moments in cell division – a complex and crucial process of normal cell growth and tissue maintenance.
It then outlines evidence that cannabis disrupts this process at specific points, leading to potentially cancer-causing DNA mutations.
This is a relatively recent discovery known as chromothripsis, which in a literal Greek translation means "chromosomes shattering into pieces".
Some of the main points revolve around the effects of cannabis on cancer and foetal abnormalities.
It also touches on the possibility that genetic mutations may be passed down through generations – meaning a child who has never touched cannabis could be negatively affected because of their parents' past use.
Cannabis and cancer
The review describes several observational studies linking cannabis to cancer, including brain, prostate and lung. Many also showed the higher the cannabis use the higher the cancer risk, a tentative sign of causation.
The authors acknowledge that other studies showed no link, but suggest this might be because the participants were quite low cannabis users, making a link easier to detect, or that the link only exists after a certain threshold is passed.
For example, one study reported "heavy cannabis use" as more than 0.89 joints in one day, which may not have been enough to cause DNA damage.
Cannabis and foetal abnormalities
The review discusses several studies showing a positive link between cannabis use and foetal abnormalities such as spina bifida or low birth weight as a result of disruptions in cell growth.
As before, the authors pointed out that harms were generally found when cannabis use was high (around 50-300mg/kg) – although the definition of this was variable.
Other addictive substances
The review says that the effects of other addictive substances – alcohol, opioids, tobacco and benzodiazepines – on the development of tumours and foetal abnormalities are similar to cannabis. In other words, they all disrupt the cell cycle in a similar way.
The harmful link between alcohol consumption and tobacco use during pregnancy has long been known.
Cannabis use and future generations
Transmission of cannabis-related genetic damage from parent to child has been shown in rat and human studies, as well as for damage caused by alcohol, cocaine and opioids.
As this type of research has only just scratched the surface, the review authors said it was "an exciting time" for continuing research in this area.
How did the researchers interpret the results?
The researchers concluded that cannabis use was likely associated with cancers and other serious illnesses because it causes DNA damage in a person's cell during and around cell division.
The authors highlighted that this was an important finding as the use of cannabis is increasing globally, as is the strength of cannabis, while many countries are starting to legalise its use.
This review presents a useful summary of evidence backing the idea that cannabis can disrupt cell division, causing genetic damage, potentially leading to the development of cancer and foetal abnormalities.
The review was transparent in exploring the evidence behind one theory. And while this is a valuable body of research, a systematic review would have been more reliable, providing a more balanced view of the evidence.
Because of the uncertainty about how the included studies were chosen, there is a possibility that not all the relevant research was considered.
The strength of the included evidence was also not discussed. So we don't know whether it was generally strong or weak, or how it stacks up against counter-evidence. Results are only as good as the studies included, and this can vary depending on study design and assessment.
This type of study serves to stimulate debate and further research. It is not systematic or reliable enough to form the foundation of policy change on its own.
Read more about the potential harms of cannabis use.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Mail Online, 24 May 2016
Links to the science
Mutation Research: Fundamental and Molecular Mechanisms of Mutagenesis. Published online May 4 2016