Cannabis smoking poses a 20-times greater risk of lung cancer per cigarette than tobacco smoking yet most users of the drug are unaware of its dangers, a report says.
The UK's most popular illicit recreational drug is used by more than a third of people under 24, but 88 per cent believe it is less dangerous than tobacco. One in three 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".
"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," said Dame Helena Shovelton, BLF chief executive.
"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 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.
Cannabis smoking has been linked with a wide range of respiratory problems, while the concentration of tetrahydrocannabinol (THC) - the drug's psychoactive ingredient - has doubled since the 1990s, according to analysis of samples from police seizures.
Unlike tobacco, cannabis does not contain nicotine and so is not addictive.
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, so the long-term effects of the drug are still not known.