Occasional teen cannabis use linked to drug taking later in life
Teenage pot smokers are more likely to go on to harder drugs like cocaine and heroin - even if they have just the odd spliff, say scientists.
Smoking cannabis less than once a week was linked to a greater risk of substance abuse in early adulthood - just like those who smoke it more than once a week.
A study of more than 5,000 adolescents in England found one-in-five fitted this profile.
And the pattern was also associated with harmful drinking and smoking, said scientists.
The findings published in the Journal of Epidemiology & Community Health add to the debate about whether marijuana acts as a 'gateway' to other drug use.
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Evidence has been inconsistent so researchers looked at data from the Avon Longitudinal Study of Parents and Children (ALSPAC) - an ongoing project tracking the health of children born in South West England in 1991 and 1992.
Dr Michelle Taylor, of Bristol University, said: "One fifth of the adolescents in our sample followed a pattern of occasional or regular cannabis use, and these young people were more likely to progress to harmful substance use behaviours in early adulthood."
She said the study supports government strategies to "reduce cannabis exposure in young people."
Dr Taylor said: "We were able to look at cannabis use at six timepoints across adolescence and found 80 percent were non-users.
"Of the other 20 per cent only a small proportion were regular, heavy users but they were 37 times more likely to be tobacco dependent at 21, 26 times more likely to be using other illicit drugs and three times more likely to have harmful levels of alcohol consumption."
She said the 'gateway hypothesis' supports a sequence to drug use with alcohol and tobacco fuelling movement onto cannabis and then harder drugs like cocaine and heroin.
Dr Taylor said: We cannot say whether or not cannabis is a gateway drug yet. Hopefully we will one day."
But she said it's possibe cannabis "could alter your brain that primes you to take other drugs."
She said: "The main message is just because there's no solid evidence cannabis is a gateway drug that does not mean it isn't."
Her researchers also discovered a gender difference with boys more likely to be regular cannabis smokers - although just as many girls were occasional users."
In a bid to shed light on the issue her team looked at patterns of cannabis use during adolescence and whether these might predict other drugs abuse by the age of 21.
Information on cannabis use was available for 5,315 participants aged 13 to 18 for three or more years with complete data for 1,571 of them.
At each time point they were asked to categorise their patterns of use as none, occasional (less than once a week) or frequent (once a week or more).
Eight in ten said they were non users with 3.5 per cent describing themselves as regular users and the rest occasional..
When they reached 21 they were asked to say whether and how much they smoked and drank - and if they had taken other illicit drugs during the previous three months.
Some 462 reported recent use with 176 (38%) using cocaine, 278 (60%) 'speed' or amphetamines, 136 (30%) inhalants, 72 (16%) sedatives, 105 (23%) hallucinogens and 25 (6%) opioids.
The researchers took into account a host of potential contributory factors and found being male and having a mother who was a substance abuser were strongly associated with cannabis use during adolescence.
Smoking, drinking and having behavioural problems before the age of 13 also added to a participant's risk of smoking dope.
The study found those who used cannabis in their teens were at greater risk of problematic substance misuse by the age of 21 than those who didn't.
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 early and late onset occasional cannabis use during the teenage years were also associated with 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 and other substance misuse by the age of 21.
Dr Taylor said: "Our study does not support or refute arguments for altering the legal status of cannabis use - especially since two of the outcomes are legal in the UK.
"This study and others do, however, lend support to public health strategies and interventions that aim to reduce cannabis exposure in young people."