Smoking cannabis can have a devastating effect on the heart, new research has warned.

Doctors say the risk increases by almost five times in the first hour after the drug is smoked.

They cite the case of a 21-year-old man who had a heart attack after smoking cannabis - and say the drug was to blame.

The report, published in the Journal of Emergency Medicine, described the man as a regular marijuana and cigarette smoker.

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Doctors warn smoking cannabis might trigger heart attacks, after the drug was blamed for an attack suffered by an otherwise healthy 21-year-old man

He arrived at the emergency department at the University Hospital Wales in Cardiff complaining of a sharp pain on the left side of his chest that lasted for half an hour.

He said the pain started after he played a game of football.

A month before, doctors at the same hospital had diagnosed him with muscle pain after he went to the emergency department complaining of the same symptoms after playing football.

But this time, tests showed his triglyceride and cholesterol levels – fats in the blood - were much higher than normal.

He was rushed to the nearest cardiology centre, where an X-ray of his coronary arteries revealed a blood clot blocking the heart’s blood supply, which resulted in a heart attack.

Doctors said cannabis use was the most significant cause of the ‘acute coronary syndrome’ he suffered.

This is the medical term for the symptoms which occur when the coronary artery is blocked.

The man had no other risk factors for cardiovascular problems, and although he had used cocaine four months previously, this was judged to be too long ago to have caused it.

Doctors said it is not known how smoking marijuana can cause a heart attack, but that it is known to affect blood flow, increase heart rate, cause high blood pressure when sitting down and low pressure when standing up.

The report backs up previous research which found that smoking marijuana can trigger heart attacks in the young and middle-aged.

French researchers warned that people with pre-existing heart weaknesses are at greatest risk.

The risk of a heart attack increases by almost five times in the first hour after smoking, doctors said

Writing in the Journal of Emergency Medicine, the Welsh medics said: ‘Although our patient was a cigarette smoker and had elevated lipid levels, cannabis use was identified as the most significant precipitant of his acute coronary syndrome (ACS).’

They added there is evidence cannabis seems to be a rare trigger factor for heart attacks, with the risk increased by almost five times in the first hour after smoking it.

They pointed to other cases where doctors said cannabis had caused heart problems, and advised that medics should ask about cannabis use when patients complain of chest pain or have heart-related problems.

The researchers concluded: ‘There is an increasing amount of data to suggest an association between cannabis use and ACS, though no specific mechanism has yet been established.