Cannabis users are up to five times more likely to have suicidal thoughts than non-smokers, a new study shows.
The class B drug has long been anecdotally linked with mental health problems, including depression and anxiety.
But a study of more than a quarter of a million young adults in the US, run by the National Institutes of Health, assessed the strength of the connection.
It revealed that around 3 per cent of the population who do not smoke cannabis and do not suffer from depressive episodes have some suicidal thoughts.
But the prevalence of suicidal thoughts increased the more often a person smoked a joint.
For example, seven per cent of people who smoked cannabis, but less than once a day, reported suicidal thoughts, rising to nine per cent for those who smoked the drug every day.
But 14 per cent of people with a diagnosed cannabis use disorder, who often smoked the drug several times a day, reported suicidal feelings, a five-fold increase compared to non-smokers.
The figures were higher for people who already suffered from depression with one in three (35 per cent) non-smokers with the mental illness having thoughts pertaining to suicide.
Cannabis use disorder
This already elevated baseline figure then rises further to 44 per cent in those who use cannabis, to 53 per cent of those who use cannabis daily, and to 50 per cent in people with "cannabis use disorder".
"While we cannot establish that cannabis use caused the increased suicidality we observed in this study, these associations warrant further research, especially given the great burden of suicide on young adults," said Dr Nora Volkow, senior author of the study from the National Institute on Drug Abuse (NIDA) in Maryland.
"As we better understand the relationship between cannabis use, depression, and suicidality, clinicians will be able to provide better guidance and care to patients."
The researchers also looked at how smoking marijuana impacted men and women differently, and discovered female users were more prone to problematic thoughts.
Data show that around one in seven (13.9 per cent) women who have no track record of depression but have a cannabis use disorder have suicidal leanings. However, this figure is just 9.9 per cent for men.
Among individuals with both cannabis use disorder and major depressive episodes, the amount of people with suicidal thoughts was 52 per cent higher for women (23.7 per cent) than men (15.6 per cent).
"Suicide is a leading cause of death among young adults in the United States, and the findings of this study offer important information that may help us reduce this risk," explained lead author Dr Beth Han, from NIDA.
"Depression and cannabis use disorder are treatable conditions, and cannabis use can be modified.
"Through better understanding the associations of different risk factors for suicidality, we hope to offer new targets for prevention and intervention in individuals that we know may be at high risk.
"These findings also underscore the importance of tailoring interventions in a way that take sex and gender into account."
Suicide was the cause of 5,619 deaths in England and Wales in 2019 and 4,303 of these were men, according to data from the Office for National Statistics.
Men aged between 45 and 49 experienced the highest number of suicides, accounting for 25.5 deaths per 100,000 males.
Figures from 2020, a year ravaged by the coronavirus pandemic, have not yet been released due to the long delay in collecting suicide statistics.
But despite initial concern the lockdowns would see an increase in suicides, preliminary data indicates 2020 is likely no worse than 2019.
The findings of the new study are published in the medical journal JAMA Network Open.