A cannabis expert today warned of the growing dangers of the drug but attacked misleading reports that it was as addictive as heroin.

Professor Wayne Hall said he was increasingly concerned at the increased potency of the drug and its use by children in their teens rather than something first tried by university students.

He said cannabis remained less addictive than alcohol, nicoteen, heroin and cocaine and said it was “not helpful” to have it compared to Class-A drugs in media reports today.

“The intention was not to suggest that cannabis was as harmful or as risky as heroin,” he told the Standard. “It was to make the point there are risks with the use of the drug.

“It’s a drug of dependence like alcohol, nicotine and heroin but not as high a risk.”

His paper, which analysed worldwide research into drug addictions since 1993, said that driving while under the influence of cannabis doubled the risk of a car crash while its use during pregnancy reduced the birth weight of the baby.

Prof Hall, of King’s College London, said the risk of regular users developing schizophrenia in adulthod had increased over the last 20 years from one per cent to two to three per cent.

He said: “As a broad generalisation, the earlier you introduce the use of any drug, the greater the risk. There is probably a bigger risk It’s a combination of a more potent product and earlier initiation that has produced that increase.”

He quotes US research from the early 1990s that the life-time risk of developing dependence on substances ranges from nine per cent for cannabis to 15 per cent for alcohol, 23 per cent for heroin and 32 per cent for nicotine.

He writes that the “evidence for a cannabis withdrawal syndrome has strengthened since 1993” and “it is now difficult that cannabis dependence does not require medical attention”.

He added: “The adverse health and social consequences of cannabis use reported by cannabis users who seek treatment for dependence appear to be less severe than those reported by alcohol and opioid-dependent people.”