"Cannabis: the terrible truth," is today's Daily Mail front page splash story. The paper cites the risks posed by cannabis – including a doubling of the risk of schizophrenia – based on research the paper says has "demolished the argument that the drug is safe".

The "terrible truth" is we still don't know enough about the safety and harms of cannabis because it's legally and ethically a difficult area to research. However, we can be pretty certain you can't take a fatal overdose from recreational cannabis use.

The headlines in the Mail and several other papers were prompted by the publication of a narrative review of cannabis research by Professor Wayne Hall, an expert adviser on addiction to the World Health Organization.

Professor Hall concludes that cannabis research since 1993 has shown its use is associated with several adverse health effects, including a doubling of the risk of crashing if driving while "cannabis impaired". He also found that around one in 10 regular cannabis users develop dependence.

He also reports regular cannabis use in adolescence was strongly linked with using other illicit drugs, as well as increased risk of cognitive impairment and psychoses.

In addition, cannabis smoking probably increases cardiovascular risk in middle-aged adults with pre-existing heart disease, but its effects on respiratory function and respiratory cancer remain unclear as most cannabis smokers have smoked, or still smoke, tobacco.

But as this review was not systematic, it is impossible to tell if all relevant studies have been included. And all these conclusions were based on the results of observational studies, which means we can't tell if cannabis caused all the effects.

Where did the story come from?

The study was carried out by a single researcher from the University of Queensland Centre for Youth Substance Abuse Research, the University of Queensland Centre for Clinical Research, and the National Drug and Alcohol Research Centre in Australia, and the National Addiction Centre at King's College London.

It was funded by the National Health and Medical Research Council of Australia and was published in the peer-reviewed journal, Addiction.

Despite the somewhat hyped headlines, the media coverage of this study was generally accurate, but did not point out the limitations of the research. Indeed, the Mail's description of the study as "definitive" is rather at odds with the nature of the research.

What kind of research was this?

This was a narrative review that aimed to examine the changes in the available evidence on the adverse health effects of cannabis since 1993.

It was not clear how the author identified the studies used as a basis for the review. It may be the case there are other studies showing no effect or harm that have not been included in the review.

It is also not clear how the author compiled the results of the research to come up with strengths of effect.

A systematic review is required to assess the adverse health effects of cannabis use.

Also, although the author applied rules to the interpretation of the research, the conclusions are based on the results of observational studies.

It is difficult to conclude from these types of studies that cannabis causes the effects seen, as there are still potentially differences between people who use cannabis and people who don't that could explain the differences seen.

What did the research involve?

The author looked at studies published over a 20-year period since 1993 (when a previous review was conducted) to see if there was evidence that cannabis caused adverse health effects. To do this, Professor Hall looked at whether:

there were case control and cohort studies that showed an association between cannabis use and a health outcome

cannabis use preceded (started before) the outcome

the association remained after controlling for potential confounding variables

there was clinical and experimental evidence that supported the biological plausibility of a causal relationship

What were the basic results?

The author listed the conclusions that he believes can now reasonably be drawn in the light of evidence that has accrued over the past 20 years.

Adverse effects of acute use

Professor Hall concluded that:

The risk of a fatal overdose is considered to be extremely small. The estimated fatal dose in humans is between 15 and 70g, far greater than it is reported a heavy user could ever use in one day. There have also been no reports of fatal overdose in the literature.

Driving while cannabis impaired approximately doubles car crash risk.

Maternal cannabis use during pregnancy modestly reduces birthweight.

Adverse effects of chronic use

Professor Hall concluded that:

Around one in 10 regular cannabis users develop dependence, and this rises to one in six among people who start in adolescence.

Regular (daily or near daily) cannabis use in adolescence approximately doubles the risks of early school leaving and cognitive impairment and psychoses in adulthood.

Regular cannabis use in adolescence is also associated strongly with the use of other illicit drugs.

Cannabis smoking may increase the risk of cardiovascular events such as angina or heart attack in middle-aged and older adults with pre-existing cardiovascular disease. Some isolated reports suggest younger people not yet diagnosed with cardiovascular disease may also be at risk of cardiovascular events.

The effects of cannabis on respiratory function and respiratory cancer remain unclear because most cannabis smokers have smoked, or still smoke, tobacco.

How did the researcher interpret the results?

Professor Hall concluded that: "The epidemiological literature in the past 20 years shows that cannabis use increases the risk of accidents and can produce dependence, and that there are consistent associations between regular cannabis use and poor psychosocial outcomes and mental health in adulthood."

Conclusion

This narrative review has concluded that cannabis research in the past 20 years has shown that cannabis use is associated with a number of adverse health effects.

It also found driving while cannabis impaired approximately doubles car crash risk and around one in 10 regular cannabis users develop dependence.

Regular cannabis use in adolescence approximately doubles the risks of early school leaving and cognitive impairment and psychoses in adulthood, according to the review.

Regular cannabis use in adolescence is also associated strongly with the use of other illicit drugs.

In addition, cannabis use probably increases cardiovascular risk in middle-aged adults with pre-existing heart disease, but its effects on respiratory function and respiratory cancer remains unclear because most cannabis smokers have smoked, or still smoke, tobacco.

However, as this was not a systematic review it is impossible for readers to know whether all relevant studies have been included.

All the review's conclusions were based on the results of observational studies. So while it seems probable that cannabis use increases the risk of some adverse outcomes, it is also possible there are differences between cannabis smokers and non-smokers that explain some of the differences seen.

Analysis by Bazian

Edited by NHS Website