File photo:It also discovered that teenage cannabis smokers are 37 times more likely to be hooked on nicotine and three times more likely to be problem drinkers than non-users of the drug. Picture: REUTERS

Teenagers who regularly smoke cannabis are 26 times more likely to turn to other drugs by the age of 21.

The study of the lives of more than 5 000 teenagers produced the first resounding evidence that cannabis is a gateway to cocaine, amphetamines, hallucinogens and heroin.

It also discovered that teenage cannabis smokers are 37 times more likely to be hooked on nicotine and three times more likely to be problem drinkers than non-users of the drug.

The findings from Bristol University provide authoritative support for those warning against the liberalisation of drugs laws. Medical researchers have argued for years that cannabis is far from harmless and instead carries serious mental health risks.

Dr Michelle Taylor, who led the study, said: "It has been argued that cannabis acts as a gateway to other drug use. However, historically the evidence has been inconsistent.

"The most important findings from this study are that one in five adolescents follow a pattern of occasional or regular cannabis use and that those individuals are more likely to be tobacco dependent, have harmful levels of alcohol consumption or use other illicit drugs in early adulthood.

"Our study does not support or refute arguments for altering the legal status of cannabis use. This study and others do, however, lend support to public health strategies and interventions that aim to reduce cannabis exposure in young people."

The Bristol evidence was gathered from a long-term survey of the lives of young people around the city, the Avon Longitudinal Study of Parents and Children.

The survey, which was published in the Journal of Epidemiology & Community Health, examined 5 315 teenagers between the ages of 13 and 18. One in five used cannabis.

Dr Tom Freeman of King’s College London said: "This is a high quality study using a large UK cohort followed from birth. It provides further evidence that early exposure to cannabis is associated with subsequent use of other drugs."

Ian Hamilton, who is a mental health researcher at York University, said: "It adds to evidence that cannabis acts as a gateway to nicotine dependence, as the majority of people using cannabis in the UK combine tobacco with cannabis when they roll a joint.

"This habit represents one of the greatest health risks to the greatest number of young people who use cannabis.

"It suggests that adolescent cannabis use serves as a gateway to a harmful relationship with drugs as an adult."

The report said: "After taking account of other influential factors, those who used cannabis in their teens were at greater risk of problematic substance misuse by the age of 21.

"Teens who regularly used cannabis were 37 times more likely to be nicotine dependent and three times more likely to have a harmful drinking pattern than non-users by the time they were 21. And they were 26 times more likely to use other illicit drugs.

"Both those who used cannabis occasionally early in adolescence and those who starting using it much later during the teenage years had a heightened risk of nicotine dependence, harmful drinking, and other illicit drug use. And the more cannabis they used the greater was the likelihood of nicotine dependence by the age of 21."