A third chemical – cocaethylene – builds up in the liver over a number of years among those who mix the two drugs. And this is now having major health consequences

"I first took coke when I was 18 and at university. I remember two friends who did chemistry told me I should get really drunk first because it would mix into this new chemical in my blood and make me even higher," a 30-year-old woman who works in publishing told the Observer yesterday.

What her friends did not tell her is that the combination of cocaine and alcohol in her then teenage body will have left a highly toxic chemical in her liver called cocaethylene.

While few outside the world of pharmacology have heard of the chemical, fewer still are aware of its life-threatening properties. Now, however, its side-effects, discovered in 1979, are threatening to become tragically familiar as they take their toll on users in their 30s and 40s.

Drug addiction clinics say they are becoming increasingly concerned by the health risks associated with the chemical – the only known example of the body forming a third drug following the ingestion of two others.

For not only is cocaethylene toxic in the liver, it is also blamed for heart attacks in the under-40s and a surge in social problems. But because so little is known about the drug, few experts can agree on the nature of the threat to users, and indeed society as a whole.

Many who consider themselves social users – for whom a line or two of cocaine coupled with a few drinks in a bar or a club is a weekly or monthly event – do not consider the health implications of their drug taking.

"I am not sure I have ever taken coke when I haven't been drinking alcohol," one 30-year-old television producer who has been taking the drug socially for the past seven years said yesterday. "It allows you to drink more, so if I am feeling a little too drunk I might take a line as a sharpener. It makes you feel a bit more sober."

The woman, from west London, who asked not to be named, said that she used to take the drug much more frequently than she does now: "At the peak I would probably do it with alcohol every weekend for a few months and then have a few months off. But I don't take as much now because the quality has dropped."

She said that a "friend of a friend" who took the drug regularly and also drank had suffered a heart attack at the age of 36. Although he had survived, he had needed serious surgery.

"I don't really worry though," added the woman. "Partly because I don't do so much now, but also I do lots of other risky things. I smoke. I guess they are all calculated risks. They are dangerous but I don't see the point in worrying about them."

Talk to the people on the frontline of the drugs war, however, and they have no doubt the mixing of cocaethylene's two ingredients is becoming a troubling social issue.

Karen Colgan, who helps run a treatment agency for the charity Addaction, in Lincolnshire, said the trend was being driven by a new generation of users. "We are seeing more young people using more substances," she said. "They don't see it as a problem, because they are just using at weekends.

"They do it to increase the effects of cocaine, but we have found cases where young people are getting heart palpitations and then going and playing Sunday football."

Increasing cocaine use among the young may explain heightened concerns about the effects of cocaethylene. Last year's British Crime Survey revealed that there had been a 25% increase in the number of 16- to 24-year-olds taking the drug compared with the previous year. The number of people under 25 who have sought help for cocaine abuse has doubled in the past four years.

But the effects are being felt by all demographics. Figures obtained by the Liberal Democrats reveal that the number of people going to hospital after overdosing on cocaine has doubled over the past five years, with one person now admitted every 10 hours. Alcohol is thought to play a significant part in many of these admissions.

Cocaine-related deaths are also increasing in the US. The US National Household Drug Survey estimated that around five million people used alcohol and cocaine each month.

"There is no question that the use of both alcohol and cocaine is a growing concern," said Martin Barnes, chief executive of the charity DrugScope, who is concerned about how to address the trend. "Surveys show people who drink regularly in bars and clubs are more likely to be using alcohol and drugs. This presents challenges to health professionals about how to raise awareness of the health risks because combining the two drugs is not seen as taboo. It's simply a lifestyle choice."

Cocaethylene's powerful effects may help to explain the increasing attraction of combining alcohol and cocaine. A 2003 study by scientists at the University of California's Drug Dependence Research Centre noted: "The combined use of alcohol and cocaine can produce a sense of increased and prolonged euphoria, compared with the use of either substance on its own."

This claim is disputed by some scientific studies. But what seems irrefutable is that the two drugs can "stretch the good times", as one regular cocaine and alcohol binger told the Observer. "I can drink for ever on coke," said Dave, a 28-year-old electrician from Manchester. "Without it, things can go downhill very fast."

His comments are backed up by independent research. The respected magazine Druglink reported that a 2006 analysis of 102 alcohol and cocaine users, carried out by the UK National Addiction Centre, found strong links between snorting cocaine and long, heavy drinking sessions. Almost half of regular powder cocaine users questioned for the analysis said that their last heavy drinking episode had lasted more than 12 hours.

The ability of cocaine users to consume vast amounts of alcohol is being blamed for an increase in sexually risky behaviour among the young and rising levels of violence. Small studies in Manchester and Merseyside suggest that around half of all young people arrested for violent behaviour were on drugs, and of these the majority were on cocaine. Many had been drinking prior to their arrest.

Greater Manchester's chief constable, Peter Fahy, has also observed the trend to combine cocaine and alcohol. "I am concerned that we seem to be producing a lot of angry young men at the moment," he said last year. "We need to understand why that is, and why some of these young men are quite prepared to use extremes of violence over nothing. I think alcohol plays a part, but we are also concerned about the mixture of alcohol and cocaine."

But while there is emerging evidence about the social effects of combining alcohol and cocaine, there is little research into the physiological effects of cocaethylene on the body.

There is a growing suspicion that the drug may be at least partially responsible for the perceived increase in the number of people in their 30s suffering heart problems. According to US Drug Abuse Warning System, "cocaine/ethanol abuse is a major cause of emergency medical admissions" and "the cause of increases in cocaine-related mortality".

Steven Cox, deputy head of the charity Cardiac Risk in the Young, said around 12 fit young people are recorded as dying each week in the UK from a cardiac-related illness, compared with about four a week in 1995.

But Cox said this increase might be down to a better analysis of postmortem data, which has raised awareness of cardiac-related illnesses, rather than any external factor.

However, he said that an estimated one in 300 people who have been tested by the charity is found to have an underlying heart condition. "We do know that if you have an underlying cardiac condition certain drugs can trigger an effect," Cox said. But until someone proves the link, it seems unlikely that the trend for people to mix alcohol and cocaine will show any sign of abating.