Smoking cannabis causes structural damage to the heart which may increase your risk of a stroke, research suggests.
Regular users of the drug were found to have an enlarged left ventricle, the heart’s main pumping chamber.
As its walls get thicker less blood is able to be transported around the body, which experts say can increase the risk of strokes.
Regular cannabis smokers also had impaired heart function, with tests showing their muscle fibres deforming whenever their hearts contracted.
Scientists from Queen Mary University of London, who led the study, have called for urgent research on the safety of cannabis on heart health.
Smoking cannabis may cause potentially lethal damage to the heart, a study suggests (stock)
They analysed MRI scans from 3,407 people in the UK Biobank. The average age was 62, and none of them had any known heart issues.
A total of 3,255 of the participants said they rarely or never used cannabis, while 47 were current regular users.
The remaining 105 had used the drug previously, but not for five years before the study began.
Current smokers were more likely to have larger left ventricles and showed signs of impaired heart function.
No changes were identified in the size and function of the other three chambers of the heart.
The analysis also found that people who had quit cannabis had similar heart size and function to those who’d never used the drug.
The Mayo Clinic, a US health organisation, says an enlarged left ventricle can lead to strokes and heart failure. But the new research did not look at how common strokes were.
Although the study took account of factors such as age, diabetes, blood pressure, smoking and alcohol consumption, the authors acknowledged it had limitations.
These included the over-representation of Caucasian participants (96 per cent), the relatively low number of regular cannabis users and reliance on self-reported usage.
They note that volunteers may have been reluctant to admit to using it because it is illegal in the UK.
Sceptics also say cannabis smokers tend to have higher blood pressure, which may have raised their risk of an enlarged heart.
High blood pressure is a known cause of an enlarged left ventricle because it forces the pumping chamber to work harder to push blood around the body.
Cannabis users are more likely to drink heavily and smoke tobacco – both habits are known to raise the risk of hypertension.
Dr Mohammed Khanji, lead study author and senior clinical lecturer at Queen Mary, said: ‘Our findings are not conclusive.
‘But the research took place against a backdrop of decriminalisation and legalisation of recreational cannabis use in many countries.
‘We urgently need systematic research to identify the long-term implications of regular consumption of cannabis on the heart and blood vessels.
‘This would allow health professionals and policymakers to improve advice to patients and the wider public.’
Professor Jeremy Pearson, Associate Medical Director at the British Heart Foundation, said: ‘We now urgently need a much larger study looking at how cannabis use impacts our heart health, so that people who do choose to use it can be made fully aware of any and all adverse effects.’
The results are published today in JACC Cardiovascular Imaging.
The World Health Organisation (WHO) has warned about the potential harmful health effects of smoking cannabis and called for more research around its cardiac impact.
A study published last month by the George Mason University in Fairfax, Virginia, found smoking cannabis doubled the chance of a stroke.
Researchers looked at more than 40,000 adults aged between 18 and 44 over a year-long period.
They found those who used cannabis more than three times a week were two-and-a-half times more likely to have a stroke compared to non-smokers.
Previous research has shown compounds in the drug known as cannabinoids may cause blood vessels to narrow.
Eleven US states have already legalised the recreational use of marijuana, including California, Colorado and Massachusetts.
Cannabis can also be taken for medical reasons in 21 additional states, such as Illinois, Florida and Pennsylvania.
It comes a year after medical cannabis, which is taken either orally or in pill form, was made legal in the UK.