Swiss scientists say hundreds of thousands of patients and drug addicts should no longer receive the most widely used form of methadone because of heart attack risks.
A study presented on Monday revealed that one of the two components of the synthetic drug is cardiotoxic and should be removed.
Methadone is used both as a painkiller and to wean drug addicts off opiates such as heroin and morphine. It was recently added to the Geneva-based World Health Organization's (WHO) list of essential drugs, yet has been linked to a number of deaths, notably in the United States.
In November last year the US Food and Drug Administration issued a public health warning, advising doctors and patients that methadone "can cause life-threatening heartbeat problems that can lead to death".
Now pharmacology and pharmacogenetics specialists at Lausanne University and Lausanne University Hospital say they have identified a way of making the synthetic drug safer.
The form of the drug currently used by most countries is a combination of 50 per cent active methadone (R-methadone) and 50 per cent inactive methadone (S-methadone). More than half a million patients receive this version worldwide.
But according to the Lausanne study of 179 patients, S-methadone is responsible for causing cardiac arrhythmias or an irregular heartbeat, although it is pharmacologically inactive.
Moreover in at least six per cent of patients with a specific genetic background, this risk is even higher because they break down the S-methadone more slowly.
"If one wants to avoid these cardiotoxic effects then one should only prescribe R-methadone," Hugues Abriel from the university's pharmacology and toxicology department told swissinfo.
This is already the case in Germany where methadone was first used as a painkiller during the Second World War. Abriel said bigger trials were now needed to confirm the findings.
"This shouldn't take long. Maybe a year or less, we hope," he said. "If this is proven, we would expect many, many thousands of people to change to the R form only."
Harmful side effects
The professor added that such a switch would lead to a minor increase in costs as the methadone mix is cheaper to produce, but would massively reduce the risk of harmful side effects.
"The cost factor is not very important. Methadone is a very cheap drug. It only costs around SFr0.80 [$0.65] a day per patient and would rise to around SFr2.5 if we just use R-methadone," he said.
Suzanne Hill, secretary of the WHO's expert committee on the selection and use of essential medicines, said the cardiac risks associated with methadone had been known for some time.
She said they had been taken into account when the drug was added to the agency's list of essential medicines and she saw no need to review the situation in the light of the findings.
However, she told swissinfo that if the research was substantiated it would open up "interesting possibilities further down the road".
swissinfo, Adam Beaumont in Geneva
The Swiss study was carried out jointly by teams from Lausanne University and Lausanne University Hospital.
It is estimated that more than half a million patients around the world use a methadone mix of the R and S versions.
The findings of the Swiss study are revealed in this week's edition of the medical journal Clinical Pharmacology & Therapeutics.
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