A US coroner has stood by his conclusion that a woman died of an overdose of THC – the active ingredient in cannabis – despite experts casting doubt on the finding.

In what would ostensibly be the first confirmed case in the country, coroner Dr Christy Montegut, of St John the Baptist Parish in Louisiana, said an unnamed 39-year-old woman died in February after vaping THC oil.

“It looked like it was all THC because her autopsy showed no physical disease or afflictions that were the cause of death. There was nothing else identified in the toxicology — no other drugs, no alcohol. There was nothing else,” Dr Montegut told the New Orleans Advocate newspaper last month.

He added: “I’m thinking this lady must have vaped this THC oil and got a high level in her system and (it) made her stop breathing, like a respiratory failure.”

The woman’s toxicology report said she had 8.4 nanograms of the substance per millilitre of blood in her system, which Dr Montegut said was likely the result of vaping using highly concentrated THC oil.

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Colorado, which legalised recreational marijuana in 2013, set the legal limit for driving while impaired by cannabis at 5 nanograms per millilitre.

The woman's boyfriend said she had been sent to hospital three weeks before she died with a chest infection. Doctors, he said, sent her home with over-the-counter medicine.

Dr Montegut later doubled down on his conclusions, telling TV channel WWL-TV he was “100 per cent sure of the readings we’ve found”.

“I definitely did some research before I came to the conclusion that this was the cause of death,” he said.

But drugs experts were sceptical.

Steve Rolles, senior policy analyst at the Transform Drug Policy Foundation, told The Independent drug death attribution was a "minefield" and that the presence of a drug in a person's system did not prove it was their cause of death.

He said there had been cases in the past where people had died of coronary events after using cannabis, but that causation was difficult to establish.

Professor David Nutt, the UK government's former drugs czar, speculated the woman could have died due to increased blood pressure and heart rate, a phenomenon the scientist said could be caused by THC through "an as yet not understood mechanism".

"Its use is not advised in people with heart problems like angina for this reason," he added.

Even if causation is established, Mr Rolles said it did not "really change the equation" in terms of mortality risk for cannabis users.

“A death out of 250 million users globally - it still doesn’t really change the calculus in terms of mortality risk," he said.

"Cannabis is very low on the scale."

Keith Humphreys, a former policy adviser at the White House Office of National Drug Control Policy, told the Advocate that if THC were actually toxic at consumable levels there would likely be many overdose deaths.

“We know from really good survey data that Americans use cannabis products billions of times a year, collectively. Not millions of times, but billions of times a year,” Mr Humphreys said.

“So that means that if the risk of death was one in a million, we would have a couple thousand cannabis overdose deaths a year.”

According to the US National Institute on Drug Abuse, there have been no reports of teenagers or adults dying from cannabis alone, though it can on occasion produce an extreme psychotic reaction.

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In Louisiana itself, state Department of Health spokesperson said all other deaths he had seen recorded with mentions of THC involvement were combined with another drug.

The controversy comes amid a push by an increasing number of US states to allow citizens to consume cannabis in small amounts.

The Illinois state House last month published a plan that would make it legal for those 21 and older to buy the drug at licensed dispensaries.

If passed, the state would become the 11th to have legalised the drug for recreational use.

THC, or tetrahydrocannabinol, is the main psychoactive component of cannabis, meaning it is the molecule which causes the psychoactive “high”.