It is widely seen as the safest recreational drug; less harmful than tobacco and less risky than alcohol. Its mellow image has led to periodic campaigns for it to be legalised. But the truth about cannabis is grimmer, experts say today. A cannabis "joint" poses a 20 times greater risk of lung cancer than a tobacco cigarette yet most users are unaware of its dangers, a report says.
The most popular illicit drug in the UK is used by more than one in three young people under 24 but 88 per cent believe it is less dangerous than tobacco. A third of those questioned said it did not harm health , despite research linking it to respiratory, circulatory and psychiatric problems.
The British Lung Foundation, which commissioned the survey, said the findings were "alarming".
Dame Helena Shovelton, chief executive, said: "New research continues to reveal the multiple health consequences of smoking cannabis, [yet] there is still a dangerous lack of public awareness of quite how harmful this drug can be."
"Young people in particular are smoking cannabis unaware that each cannabis cigarette they smoke increases their chances of developing lung cancer by as much as an entire packet of 20 tobacco cigarettes."
She called for a public health campaign to "dispel the myth that smoking cannabis is somehow a safe pastime".
The reason why cannabis is more dangerous than tobacco, per cigarette, is thought to be related to the way it is smoked. Cannabis smokers inhale more deeply and hold it longer than tobacco smokers.
The average puff on a cannabis "joint" is two thirds larger and is held four times longer than the average puff of a tobacco cigarette. As a result the cannabis smoker inhales four times as much tar and five times as much carbon monoxide. In addition, with each puff of a joint the smoke particles become more concentrated and more harmful. Cannabis smoking has been linked with a wide range of respiratory problems including wheezing, bronchitis and lung cancer.
The concentration of tetrahydrocannabinol (THC) – the psychoactive ingredient of cannabis – has doubled since the 1990s, according to analysis of police seizures of the drug, which is now chiefly grown using intensive methods in the UK. THC has been linked with an increased risk of heart attack immediately after use and suppression of the immune system.
Concern has also focused on its impact on mental health. Doctors have been worried for a decade about the effect of the drug on a small group of vulnerable users with an inherited predisposition to schizophrenia. Critics have pointed out that most cannabis users give up in their 30s, limiting their long-term exposure, which is a crucial factor in cigarette-induced lung cancer.
Two long-term studies of the drug involving more than 100,000 people in Sweden and the US found no increase in deaths. Unlike tobacco, cannabis does not contain nicotine and so is not addictive.
Although cannabis cannot be said to be safe, it is less dangerous than the legal drugs of alcohol and tobacco in terms of harm caused. However, the generation that grew up in the 1960s was the first to use cannabis on a large scale and is too young to have been followed into old age.
The British Lung Foundation says that despite the wide use of cannabis there has been little research into its health impacts.